Pocket cards is a term for the cards dealt face down to you, and the term is mostly used in the game of poker.
For example, if you are somewhat familiar with the rules of Texas Hold’em poker, then you know that in a single hand of Texas Hold’em a player will receive two cards for themselves and, depending on how far the hand progresses, five community cards that can be used by anyone to make the strongest hand.
Those two cards you are dealt at the beginning of each hand are pocket cards.
Coincidentally, if you’ve spent some time around poker players, you must’ve heard someone talking about pocket aces, which is the strongest hand you can have in Texas Hold’em pre-flop.