That's for when you have a stereo amp, or (though I've never seen one) an AV amp with no sub out. In those cases you can take a "pre amp" output from L/R (or on an AV amp the front pair), and plug the left pre out into the L socket on the QB12, and the right pre out into the R socket.
Things like that are usually covered in the sub's manual, if you've picked it up 2nd hand without a manual I recommend going on the q-acoustics website and downloading a pdf copy, as it'll help you set it up with your amp correctly. Here's the link to the
manual