Those are all of the Apple Loops. To get them back, you'd have to reinstall the program. If you want, you can copy them to another drive/location and then change the GarageBand prefs to point there instead. Useful if you have a big external drive and a smaller internal drive. (It may also speed it up a bit to have it on a separate drive, the same way Photoshop or FInal Cut Pro are faster with scratch disks... that's just speculation, though)