Cards are basically your players. The cards you select to build your team will be the players you will play with as soon as you reach the grass and start facing your opponent (hopefully) towards victory.
You also have different positions: C (Catcher), P (Pitcher), IF (InFielder) and OF (OutFielder), as much as different rarities: common (gray), rare (blue), epic (purple) and legendary (gold)
Common cards start at level 1
Rare cards start at level 3
Epic cards start at level 6
Legendary cards start at level 9
P (Pitcher) - This is the player that you use to throw the ball into your C (Catcher) and hopefully hit a strike with.
C (Catcher) - When your opponent is playing as the batter, this is the player who’s receiving the ball from the P (Pitcher).
IF (InFielder) - Player responsible for receiving/catching the ball in one of the three bases.
OF (OutFielder) - Players that are more far from the batter, responsible to catch balls that cross the IF (InFielder), either from the ground or air. Ideally, they want to catch the ball before it hits the ground, but if it does, they are responsible for throwing the ball as fast as possible for their IF (InFielders) to have the ball in their hands next to their base.