There are six license options to choose from when uploading your music; they proceed from most restrictive (“all rights reserved,” the default) down to most permissive (“attribution”).
To edit the license of a track, click the yellow “Edit” button beneath the digital item’s title, choose a track, then scroll down to the bottom of the track editor, where you’ll find the “license” section. You can read more about each license by clicking the “info” link. Once you’ve chosen a license, it’s added to the footer of your track (or album, if all the tracks it contains have the same license) for your fans to see. More info here.