Training players allows you to increase their skill levels and effectiveness by feeding them excess or unused cards.
Player training is an option available on the Manage Team screen. Simply tap on any position to bring up the card maintenance screen for that position.
Tap on the player you want to train to bring up the player screen. From here you can tap on the player's card again to see their skills and a fun fact about the player.
If you have unused cards tap on the Train button and you will go to the Training screen.
To Train the player simply tap and hold on an unused or excess card and drag it to the player you are training. You will see how much XP the card gives the player and how many coins it will cost to train him.
Drag as many cards as you like to the player and when you have reached the training level you desire tap the train button on the bottom right corner to complete the training process. The player being trained will then absorb these cards removing them permanently from your roster.
At any point, you can tap on the back button on the bottom left corner to cancel the training process.