What does auto fill mean?

When you choose autofill for the i-Phone, it randomly chooses songs from your i-tunes to put on your i-Phone. If you go to settings, you can choose how many GBs you want of music on your phone, and the choices off it randomly downloading higher rated songs, random songs in general, or whether or not you'd like to replace the music you may have downloaded some other time or not. Hope this helped.