This is a tribute to Syd Barrett, an original member of Pink Floyd - notice the title, Shine On You Crazy Diamond. He was their lead guitarist and wrote most of their early hits, but he gradually went nuts and was kicked out of the band in 1968, three years after the group started. Drugs played a big role in his mental illness.
During the final mixing sessions of this song in June of 1975, Barrett wandered into the studios, ready to help out. He was fat, bald, and as crazy as they remembered, but they let him stay for a while. Barrett wanted to rejoin the group, but they learned in 1967 and 1968 that having an insane member was not good for a band. Before he was kicked out, Barrett would get on stage and either refuse to play or play the same note over and over.
According to the Pink Floyd autobiography A Saucerful of Secrets by Nicholas Schaffner, when Barrett came into the control room, the remaining members of Pink Floyd were listening to the finished recording of the album. This was the eve before Pink Floyd were going on a US tour. David Gilmour didn't recognize him at first - they hadn't seen him in years. Syd was fat, bald, had shaved eyebrows and was wearing a white trenchcoat with white shoes.