It's about the pressures of life, and how someone - in this case Pink - can cut themselves off emotionally from it and other people by a metaphorical wall. But, as the album shows, the problem is then one is left completely alone, something much worse than facing the things one builds the wall to get away from in the first place. Behind his wall - and with the copious amounts of drugs that Pink ingests - his mind deteriates, becoming obsessed and engulfed by all his negative emotions and, with no outlet, they turn him into a horrible, dictorial monster, until he's able to see what's happen, stop himself and, in the song the Trial, his conscious judges himself, forcing him to answer for all the choices he's made and accept responsibility that HE built up the wall, and nobody else. The last song, Outside the Wall, is ambiguous as to whether or not Pink meets a happy ending.