This is about how a Mod can't see who he really is. "Mods" were British youth who kept up with the latest music and fashion trends. Pete Townshend was a champion of Mod culture, and the rock opera Quadrophenia told the story of a Mod named Jimmy.
John Entwistle gave what many consider one of his greatest bass performances on this song. In a 1996 interview with Goldmine magazine, Entwistle explained that he recorded it in one take. He was just "joking around" when he played it, but the band thought it was great and used it in the final version.