Go to Crutchfield.com and look at the subs there.
RCA = line level = standard analog "red and white" like a tape deck, game console, vcr, etc.
speaker level = 2 wires, red and black or red and white. Just like the other speakers connected to a standard receiver.
(Many subs have both kinds of connectors, so you can wire the sub in place of the front left & right speakers using speaker wire, then attach the front left and right speakers to the sub instead of to the receiver)
receiver - --RCA / Line ---------- Sub
-or-
receiver === 2 pairs speaker wire === sub === left & right front speakers