The piston isn't pushed back into the caliper all the way. It should be almost even with the caliper when it's pushed in all the way, maybe 1/16th of an inch sticking out.
If you can't get the piston pushed in any deeper, move on to the next part:
If you still have the stock parking brake, try backing the big bolt in the center out (you'll have to loosen the lock nut first) or take the whole parking brake assembly off the caliper with the 2 allen head bolts.
If you already removed the stock parking brake setup and have a blockoff plate installed, try loosening it up or taking it off, some blockoff plates aren't deep enough and the peg on the back of the piston will hit it and keep from pushing into the caliper all the way. I ran into this problem with my moose blockoff plate, when I installed new EBC pads (thicker than stock) and a new Vesrah drilled rotor (also may be thicker than stock) I just had to get the dremel and remove a little material from both the peg and the inside of the blockoff to get it to fit back on without interfering.