In poker, the term brick is synonymous with the expression blank. It stands to describe a card appearing on the turn or the river that has no or very little connectivity with the rest of the cards and is unlikely to help any players involved in the hand.
Say the flop is 6s7hTs, and then the 2d comes on the turn. This 2d is pretty much a definition of brick in Texas Hold’em, as it doesn’t change the board in any way and is highly unlikely to improve any of the players in the pot. All of the combinations they can have now were possible before as well.
Like many other things in poker, a brick is a relative term. A card that is a brick in one scenario can be an ultimate action card in a different setup.