"I Am the Walrus" was, as Jo Grant explained, "a song by the Beatles". Jo brought it up when the Third Doctor used the phrase, "I am he, and he is me", while trying to explain how the Second Doctor was him. Jo thought he was referencing the song and replied, "And we are all together, goo goo g'joob?" The Second Doctor was unfamiliar with it but asked how it went so he could try to play it on his recorder. (TV: The Three Doctors)
Behind the scenes
The actual lyrics being referenced are from the first verse of the song: "I am he as you are he as you are me and we are all together." The phrase "goo goo g'joob" does not appear until the chorus.
While the name of song was not actually identified on-screen, the real-world "I Am the Walrus" is a Beatles song with similar lyrics. On 13 November 1972, the copyright holders of the song gave the BBC permission for the lyrics to be quoted.