Can someone explain Amps, Subs & Speakers?

Alright, so I'm re-doing my cars audio. But I want to fully understand what needs to be done before I do it. I understand that amps power subs & speakers. The terms bridge & ohms, I don't. I know mono amps have I guess one output, for a sub. 2 channel, usually for a set of speakers, 4 channels for all the speakers, and 5 for speakers & subs. Right? Now when it comes to amps pushing out power, RMS power for speakers & subs, why is that always a low number? I'm looking at a nice JL or Memphis sub with either Memphis or Infinity Kappa speakers, and an amp to push'em. If the sub pushes 200w-1200w RMS, and the amp I saw for almost $600 only pushes 300w, or something like that, how does that work? Doesn't sound like the amp is fully powering the subs potential. Shouldn't each channel in the amp push out whatever it's powering full potential? Like speakers that push 130 RMS, the amp shouldn't push 50 per channel or w/e. It should push 130 or more per channel, right? And you have a sub running to that amp, and it pushes 500w RMS, then the amp should push 500w RMS per channel to fully power the speakers and sub?