I think he means how much you can change the original tune before it starts sounding like a different song. To answer OP's question >>787
from a newb's point of view the main three aspects are:
>A: the key the song is in
Obviously if you want to rearrange it you'll have to play it in the same key or in a parallel one I believe that's what they're called please don't shoot me I started music theory just recently
>B: the main motif
Every song has a main part that makes people go "Ah ha it's THAT song"
>C: the memorable parts
Like point B, except it's other stuff, you just know them when you hear them.
I figure if you left those 3 things mostly
intact then you could tie together whatever other addition, changes and embellishments of yours without drifting too far from the original idea.