Relatability is where something in your game evokes something the player is already familiar with.
The player can relate to the item, event, or character, and they can feel like they're really in the story.
People don't actually want something new. They want to experience something they already like, but in a board game.
You convert the theme into a board game, and maybe put your own twist on it, but it's still that familiar thing.
Radlands was rethemed to post-apocalyptic. Most people love that theme. Now, they can be part of such a world.
You might say that your newfangled and unique theme is actually really good, and it will resonate strongly with the players. That's great, but are you actually going to be able to convey it? Players aren't going to read some irrelevant story from the rulebook. All you have is your artwork and names, and they have to convey the theme alone. It's really hard to teach players a whole new lore with just art and card names. They'll just see random names and artwork, and ignore it.
Magic's card sets strongly evoke things you already know — stories, themes, countries, and real history. How about ancient Japan world, with ninjas? How about an Indiana Jones type of adventure world? How about a world based on children's fairytales?
Tap into lore the players already know and love.
The Radlands expansion had a card called "bivouac". Unfortunatelly, half my playtesters didn't know that word. Some even thought it was "bio-vac". It had to change.
Your game objects should represent things the player already knows. You give a card or object a relatable name (and later, the publisher does matching artwork.)
However, this is only half the picture. The object has to actually behave like the real thing in the play of the game. The mechanics must match the theme/name of the object.
Radlands is full of such cards. There's Wounded Soldier — an excellent warrior, who starts out damaged, and you have to heal him, if you want to use him. Cult Leader sends your other people off to their doom, to destroy the opponent's stuff. Catapult lets you hurl your own people. In game terms, it destroys one of your people, and can hit anything. This matches the theme closely.
Connecting theme and function is key to immersive gameplay. If you can't do this, because your game rules are too simple, it means you need a smaller and less pretentious theme.
Also, a relatable card or item isn't just cool, it also helps people understand gameplay. A new player might be introduced to a lot of new things, but if those things work in obvious ways, it will be much easier for them to remember. An axe that cuts down trees, and gains you wood, is relatable. The Charm of Kroznor that swaps two resources for one other resource, is not relatable. (Be careful not to get it mixed up with the Charm of Grozbat, which rearranges the top four cards of the deck!)
I don't like creative names.
Radlands uses very simple names, like Scientist, Exterminator, Holdout, Sniper, Arena, and Blood Bank. These names all tie in closely to the actual function of the cards.
Some names have to be strange, because there's no simple and good name that matches the function of the card (or other game object).
However, my policy is that as many names as possible should be a "real thing". This could mean something that exists in the real world (like Catburglar, Mosh Pit, or Flamethrower.) This could also mean a known fictional thing (like Portal, Time Travel, or Magic Wand.)
If it's two words, they should be a known phrase, not simply adjective-noun. (Rabid Dog and Fuel Tanker are good. Silver Axe and Immortal Angel are not.)
Give all your cards (or other pieces) relatable names, but also work backwards from the theme. Come up with the best-known and most thematic characters, items and events, from the theme of your game. Then, make cards from them.
Magic made sets of cards set in a gothic horror world. They filled these sets with cards designed backwards from the theme. The set was full of relatable cards like Bump in the Night, Cellar Door, Boarded Windows, and Jar of Eyeballs. They introduced double-sided cards, so that some townsfolk cards could "flip over" to a werewolf side at night.
Most things in your game should be entities: people (or other characters), places, and objects. These categories are categories full of well-known and relatable things. You can certainly use actions, events, and concepts as well, but they're much harder to make relatable and different, than entities are.
In Lords of Waterdeep, you gather adventurers, and complete quests. These quests are unrelatable nonsense like "Protect the House of Wonder" and "Establish Harper Safe House". I would've themed these to be different monsters that you fight, not "quests".
In my gangster game, I gave each player a "Big Dreams" card, which was a unique and expensive thing that only they could do. I later changed this to each player having a "criminal profile". They could be a catburglar, a biker, a scientist etc. These were people, and they were much more relatable.
I've played many game prototypes about elemental magic duels. Everything is some variety of abstract spell of some sort, and it's all meaningless.
In Star Realms and Space Base, the cards are spaceships. Each card's theme only loosely relates to its mechanics. They have no character, and they all seem similar.
Wingspan is a hugely successful game about birds. However, it's just a pretty theme. It's almost impossible to connect hundreds of different birds to their abilities. What's a Canvasback anyway? (Seemingly some kind of duck.) And why is its ability "all players draw a card?"
If your game is about people and places, those people and places come from some place and time.
The real world is where you'll get the most relatable stuff. However, the real world is fairly limiting.
My gangster game fits perfectly into the real world. It uses so many real life places, and gangster tropes. It's my most relatable game.
Sci-fi is probably my next choice. It sacrifices some relatability, but isn't limiting.
Early on, there was a suggestion that Radlands could be WW2-themed. But how could I make dozens of clearly-different cards in such a world? The Mad Max post-apocalyptic theme was also suggested, and I enthusiastically endorsed it. This sub-genre of sci-fi does violence and destruction very well, and I could throw in wacky science to handle the rest.
Fantasy works, but is massively overused. Fantasy is also fairly unrelatable, except for its historical component, and all the stuff that's been used forever, like orcs and dragons.
Historical settings are a very restrictive version of the present, and are only occasionally appropriate. You would usually want to start with this theme, and build the game around it.