Definition of X
When you read poker strategy forums or chat with poker friends to analyze hands, you will see the letter X to signify a card that is not important or that you have forgotten about. For example, you read a friend's hand and he writes: "I had AJ on A-7-x". In other words, "I can't remember the third card, but it doesn't matter for the analysis of the hand. It may have been a 2 or a 3 or a 4, but it doesn't matter what card it was, it doesn't change the way we played the hand.
Other meanings of X
X also means that the card is a brick. For example, we can talk about a flop that gives a flush draw. We say "The flop was 6-7-8 with a heart flush draw". If the turn is a brick, that is, a card that does not change the nature of the flop, a card like a 2 of spades, for example, we would write that the turn was an X. X also means "check". For example, we might write that the action on the turn was x/x, in other words, pass/pass or check/check.