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 doesn't matter or has been overlooked. For example, you're reading a friend's hand and he writes: "I had AJ on A-7-x". In other words, "I don't remember the third card, but it doesn't matter when analyzing the hand". It may have been a 2 or a 3 or a 4, but no 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 color print. We say "The flop was 6-7-8 with a color print in the heart". If the turn is a brick, i.e. a card that does not change the nature of the flop, a card like a 2 of spades for example, we will write that the turn was an X. X also means "check". For example, we could write that the action at the turn was x/x, in other words, pass/pass or check/check.