noob question

i see different subs have like 2 ohm and 4 ohm options from the same kind of sub but i never understood what it meant. i see the amps have like different RMS wattages at different Ohms (2 being louder). i was wondering what this meant because it just makes me assume that i should buy a 2 ohm sub..

i am also thinking about purchasing the Alpine 15" type-r SWR-1522D. I dont know whether i should get dual 2 ohm or 4 ohm or whatever. i also dont know if i should just use a mono amp or more than one amp to power it.

thanks in advance for any help.