Definition of brick
A brick in poker is a harmless card, a card that does not change the dynamics of a board. When writing down a hand, the brick may be identified by an X ("I had AK on an A-K-x flop). The brick can be an unimportant card on the flop, but it can also be an unimportant card on the turn or river. For example, on an 8-9-J flop with two hearts, 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 in 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 what it means. For example, if you have a very dynamic board with several possible draws on the turn and the river is a brick (the card does not complete the flush or straight and does not pair the board), 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 least alert players 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.