Each player has a certain number of cards in their hand. Each card has a cost to play that is shown at the top of the card. Your hand is shown at the bottom of the battle screen.

Here you can see an example of cards in your hand. You can tell form your Player Status that you have 3 Flux and Level 2 Mountain Folk Domain. This allows you to play either of the un-greyed cards, the Axe Captain or the Craig Scout. The others either require too much flux or too high a domain level for you to use.

Discard Button: This allows you to discard a card in your hand. You wont draw a new card at the end of a round if you have 7 or more cards so this is useful if you want to get a new card. It is also useful to use if understand the autodraw feature.