Use simple language and terminology. You're just trying to help players understand the game.
Remember that even when you use simple terminology and wording, people will still fail to understand things. They'll assume things work in ways that would be clearly overpowered, they'll imagine rules that aren't there, and they'll sometimes just fail to read sentences. Don't make this even worse, by using complex wording and terminology.
The Radlands camp Reactor says it "destroys all people". I get people regularly asking if "all" is both players' people, or if it's just the opponent's people.
Do not use jargon for existing concepts that have an ordinary English word for them. Your deck is just a deck of cards, no matter how much you try to spice it up, by calling it the "archive", "spell book", or "army".
Create jargon when you create a new entity, such as a second deck. However, your jargon should be minimally creative. Its overwhelming objective should be that the greatest number of people correctly understand it. Use whatever word fits that objective best. Tell people a concept, and then ask them what they'd call it.
I like to use neutral terms like location, person/character and board wherever possible. This is even more important when you have varying types of that thing. I don't want to have to remember which guys are "allies", which are "mercenaries" and which are "heroes".
However, you should avoid using known concepts, from classic games. This includes words like joker, wild, and run. These are themeless and immersion-breaking, reminding the player that they're actually just moving bits around, and not participating in an exciting world.
In Egypt-themed Egizia, there's a worker with a white skirt. He's the only worker that can add his strength to another worker. The game calls him the "joker". When I teach people the game, I call him the "helper". It's far more explanatory, and is thematic.
My publisher renamed Radlands' locations to Camps, and citizens to Punks. These added theme, while not being confusable.
Do not include Magic terminology like target (something of your choice), resolve (happen), or Dungeons & Dragons terms like 3d6 (three six-sided dice.) Don't say things like "active player" or "orthogonal" (adjacent, but not diagonally.) Game enthusiasts know these terms. No one else does.
Remember that things like resolve (happen), search your deck, and reveal a card, are unknown concepts to many people.
I avoid the terms supply and bank to refer to unused tokens or resources. People don't need to be told that unused pieces should just sit in a pile on the table, and they know that when they gain something, they get it from that pile (not from another player, for instance.)
Do not create a named "zone" for everything, that you refer to, and that players will need to remember. Things can just be "in play" or "on the table" instead of "in the battle zone". Cards on the table, but not in play, can just be "set aside".
Just because Magic calls your deck the library, and your discard pile the graveyard, it doesn't mean you should. Magic is a game with a huge amount of depth and complexity.
As with jargon, the phraseology you use (typically on cards) should simply be that which conveys the correct meaning to the most people.
The Magic card Dead Ringers reads "Destroy two target nonblack creatures unless either one is a color the other isn't." Basically, the two creatures have to be exactly the same colour or combination of colours, and not black. This card is legendary for its incomprehensibility.
You do not get any extra points for being technically correct. Only huge games like Magic need exact terminology, because they have complicated rules, and are always creating new cards.
The Radlands card "Looter" has the damage icon, and then it says "if this hits a camp, draw a card." The word "hit" isn't defined anywhere in the game, but it's obvious what it means. Replacing it with several technically-correct words would be ugly, and cause a small number of players to not understand it.
You can be very conversational in the language you use on your cards, as long as people know what you mean. You can also be redundant.
A camp in Radlands once had the text "You can only use this ability in a turn in which you play no other abilities." Basically, it must be the only ability you use in a turn.
If you analyse it, there are temporal problems with this strange wording. When you use it, you don't technically know whether it's valid to use it or not, as you're not yet sure whether you'll use another ability later in the turn. Technically, the card's text should tell you that you can only use it if you haven't played any abilities this turn, and it should prevent you from using any further abilities this turn. Despite this technical error, everyone understood the card. There was never a problem.
If anyone is failing to understand something, it's your problem.
It's fine to put explanatory text on cards. Just some small, grey text, that people don't have to read, but that people can refer to if they get confused.
Do not cater to rules pedants, by adding extra text to your cards to make them exactly accurate. Cater to the 99% of reasonable people. I answer Radlands questions all the time, where the answer is obvious, but it would be possible to interpret them diferently.
There's a Radlands camp called Catapult. You destroy one of your own people, and damage one of the opponent's cards. The art shows Catapult hurling a punk through the air. Someone asked me whether the destruction of your own person was a mandatory cost, or part of the effect. If they activated it without having a person to throw, would it still do its damage? Come on.
These days, I tell people quite openly that the rules of Radlands are shallow, and inconsistent. Some questions will have arbitrary rulings that are good for gameplay, or match the theme. Others simply have no answer.
Write game text for ordinary people to understand, not for precision.
My gangster game contained a card called "Stash". This put two money tokens on the location that the player to your right is on.
Everyone understood what the card did, but no one understood why they'd want to play it. It just seemed wacky and useless.
To me, it was obvious. I play the card, and I put two money tokens under the player to my right. On their turn, they move off that space. I'm the next player around the table, so I'll be the first player who's able to move onto that now-free space, and collect the money.
Even competent gamers didn't understand this card.
The card now has the same text, but at the bottom of the card, there is a small line of text saying: "(Hint: Go there next turn, and collect it.)" Now, everyone understands the card.