Should I delete Apple Loops?
Make a backup copy of the loop folders you delete, so an restore them from your local copy. As léonie said, Apple Loops are not essential for the functionality of GarageBand, you just can't load them anymore into your Project.
Is it legal to use Apple Loops?
They are royalty free. Apple owns the copyright to the loops, and licenses you to use them as you see fit to create something. The trainer said "copyright-free." However, they do own the copyright and give "permission" to use them in any way. No worries.
What are Apple Loops used for?
Apple Loops are prerecorded musical phrases or riffs that you can use to easily add drum beats, rhythm parts, and other sounds to a project. These loops contain musical patterns that can be repeated over and over, and can be extended to fill any amount of time.