A pound of marijuana (or kilo of cocaine or heroin) that is compacted into a cube, thus resembling a brick. This is a process that is usually done for the purpose of shipping and/or smuggling the substance.
see: Numerous Hip-Hop lyrics pertaining to "bricks" e.g. "Bricks on consignment", "I move mad bricks in the hood/on the block", "Chopping up bricks", "Cooking bricks".