Definition of brick
A brick in poker is a harmless card, a card that does not change the dynamics of a table. When writing a hand, the brick can be identified by an X ("I had AK on an flop A-K-x"). The brick can be a harmless card on an flop, but it can also be a harmless card at the turn or on the river. For example, on an flop flop-9-J with two hearts, we notice that there are many draws. A card like a T of hearts would be the opposite of a brick since it would complete many draws. A two of spades would be a good example of a brick on this board.
The implication of a brick
It is important to understand what a brick is and its implications. For example, if you have a very dynamic tableau with several possible draws at the turn and the river is a brick (the card does not complete the suit or straight and does not pair the tableau), you will have much less success with your bluff. Often players will bet draws. When the draws don't complete and it's a brick that falls, the players with the least amount of warning will understand that you probably missed your draw and are using that brick to bluff. As a general rule, a brick is a bad card to bluff on.