An ohm is a unit of measurement, which in this case is the impedance of a speaker. If all you're doing is playing with speakers, all it is is a way to calculate how much power you'll give to each speaker. There's two formulas you use (which can be combined, but I really don't feel like doing it right now) to calculate how many ohms there are, depending on how it's wired.
(R1 is the impedance of speaker 1, R2 is speaker 2... up until RX, which is the last speaker)
Series:
R1+R2+...RX=R total. This is where you have your speakers (or subs) wired one after another, plus to minus, plus to minus the whole way through.
Parallel:
(R1+R2+...RX)/(# of speakers) = R total. This is where you've got all the plus's connected together, and all the minus's connected together.
The reason those are important is because at certain loads (ohms), an amplifyer will put out a different amount of watts. If you've got a pair of subs that are 4 ohms, and your amp is stable at 2ohms, then you should wire them in parallel to represent a 2ohm load. Therefore you get a lower overall resistance, and will get more out of your speakers.
Or if you want some decent examples....
http://www.crutchfieldadvisor.com/S-igC9ReVpdTD/learningcenter/car/subwoofers_wiring.html
