The players have contracts so they can play games. Default is 7 but you can add contracts which you can buy in the trade area. Every time a player plays a game they go down by 1 contract. Fitness has been removed for FUT 21 but exists in previous versions. Pre FIFA 21 it is gained by resting and lost by playing. Generally a player with 80 or lower fitness will be much slower in the game. You can buy fitness cards to add to individual players or to the entire squad.