When creating smartlists, this is one of the most powerful/handy operators. This allows a user to combine several selections into on smartlist. The selections may also be other playlists/smartlists.
This will select all items marked with Genre rock first. Then the mix operator selects 100 items of the selected song, seperating with 50% of them beeing of rating better than 3 (so this is those with rating 4 and 5). The other 50% will have a rating less than 1 (so these are undrated).