The song title is a quote from, and the title of, the fourth book of the Hitch Hikers Guide to the Galaxy by Douglas Adams. I think it's also a eulogy, written by Adams upon the passing of Philip K Dick, thanking him for his visions - they being referred to as fish in reference to the symbol being worn by someone in the room when PKD had his wisdom teeth extracted. The sight of the symbol triggered a vision of a past life - the first of a series of experiences that inspired his later writing. Adams would be using the word in relation to Dick's ideas generally though.
