In this screenshot, the Morrigan player has one card in their EX Pocket while Akuma has none. Morrigan's EX Pocket can be seen at the top right of the screen, below her AP.
The EX Pocket (Extra Pocket) is an extra slot players have in their hands. Both players have two slots in their EX Pockets, where they are visible to the opponent at all times during a match, however, the specific card(s) in the EX Pocket are not shown to an opponent at any time. The type of card can be known by paying attention to its shape; sharp edges for Unit Cards, and rounded edges for Action Cards.
Some Abilities make cards spawn or "go back" to the EX Pocket, in the case of the EX Pocket being full, the card that is meant to be spawned or brought back will not arrive. If Incense of Doom (BOA 091) is used, all of the opponent's EX Pocket cards will be sent to their Graveyard.
Once a card is played from the EX Pocket, another one will not be drawn in the slot. Both player's EX Pocket also start empty at the beginning of a match.