You undoubtedly understand that when you cut or copy a bit of text, place your cursor elsewhere in a document, and paste, the text following the cursor is bumped forward to make room for the pasted text. But what happens when you select notes in one of GarageBand's Software Instrument tracks and paste them into an existing track?
The notes are merged with the existing track, that's what.
I provide this information not simply so you have something to chat about when conversation flags at your next cocktail party, but also because cutting and merging tracks is helpful when building a single layered track.
Say, for example, that you've got a basic drum groove you're mostly happy with. Later you decide it needs a bit more sizzle, so you create a shaker track. You can then select all the notes in the Shaker track, copy them, move the playhead to to where you'd like to merge the shaker track with the drum track, paste, and -- how about that -- the two tracks are merged. Because drum sounds are consistently mapped to the General MIDI specification, you'll continue to hear a shaker (or something darned close) even when it's moved to a different drum kit.
(I hedge with this "something darned close" stuff because certain notes will play a different tone depending on which drum kit you pick. For example, a smacking snare sound in a rock kit may be a mellower brush sound in a jazz kit.)
As you might expect, Real Instrument (digital audio) tracks don't merge with a copy and paste. Rather, anything at the playhead is replaced by the pasted audio.