The "Imagination Song" is from the Season Eleven episodes in the "Imaginationland" trilogy.
Background
The Mayor of Imaginationland teaches Stan Marsh and Kyle Broflovski the song as a means to travel to Imaginationland.
The song is described by a government technician as a "multi-tonal resonance code sequence" needed for the "fractal converter" to function. Singing portions of the song will induce an "electro-feedback nano-response" in the correctly-calibrated imagination doorway, but only singing the song in its entirety will open a corridor stable enough for passage.
The song is primarily made up of the word "imagination" repeated over and over again in slightly different pitches at different points in the song. This is sung continuously, until Kyle says, "Are you gonna take us somewhere or not?!?"
Lyrics
Imagination
Imagination
Imagination!
Imagination
Imagination
Imagination
Imagination
Imagination
Imagination
Imagination
Imagination
Imagination
Imagination
Imagination
Imagination
Imagination
Imaginatiooooooooooon!