Kaimon

Okay, say you start on a note. Musically, the smallest space to the note right next to it is what's called a "half step". So let's say we play one note and go up one half step and play that note.

Now we'll pick another note at random, one different that we played before. We also go a half step and play that note. The two sets of notes we played are made of completely different notes, but we hear them as going up the same amount.

So it's pretty much you apply that concept to two versions of the same song with different keys. As the songs go along at the same time, the notes are different, but as the go to the next note, they go up or down the same amount of steps.