Energy is the cost to play cards.
The amount of Energy you currently have in a turn can be seen in the bottom-middle of the screen during a match.
Energy starts off at 1 for the first turn, and increases by 1 for each turn.
Eg: Turn 1 you only have 1 energy
Turn 2, you have 2 energy
Turn 6 you have 6 energy
Note: Energy does NOT carry over from a previous turn. Try to spend all the energy you can!