"Tribute" is a tribute to "The Greatest Song in the World", which Tenacious D themselves came up with, but have since forgotten.
During earlier performances of this song Kyle Gass played the opening to "Stairway to Heaven". The two songs are both in A minor and have very similar chord progressions, and critics have said the songs sound alike.
Given the "Stairway to Heaven" interlude in the original TV series version, it might be suggesting the best song in the world is indeed that song, but the lyrics make clear that the song Tenacious D is playing sounds nothing like the song they came up with to please the demon (specifically, "This is not / The greatest song in the world, no / This is just a tribute").
In an interview the band claimed that the inspiration from the song came after Jack Black played Metallica's "One" for Kyle Gass, describing it as "the best song in the world", leading to a failed attempt to themselves write an even better song, and a discussion of the meaninglessness of labeling any song that way.