Guy plays guitar, sings, plays harmonica, kazoo, drums, bells, whistles, and more all at the same time while walking down the street.
Clip from This Is Spinal Tap in which Nigel Tufnel shows off his amps that with controls that go up to 11 (rather than the standard 10) - so the band can get that extra "push over the cliff."
The phrase has it's own Wikipedia Page, becoming a popular number used to represent the maximum volume for media players online and even on some devices.
