Editing drag markers allows you to extend or compress notes, so you can drag a note out to last over a longer period of time, or you can compress the note.
Place the cursor over the peak of a note that you wish to edit.Click once to have GarageBand automatically sync the peak of the note with the timeline grid. This will make the note snap into place and be in time with the tempo of the song.
You can then drag that note out so that the sound of it lasts longer in the song. This is just like stretching out that note.