The 1 is for the subwoofer

I don't know why they use "5.1" -- it's not "five and one-tenth" but "five speakers (front left/right, rear left/right, and front center) + a subwoofer"
But whatever they call it, it sounds great!
(I think they started 6.1 now, though, with a rear center channel. Great, right after I get my 5.1 system. But I haven't seen any discs yet using 6.1 so I don't care...)
JG