[sr20de4evr]

ohms are a rating of an electical component's resistance to current. A voltage is applied across an electrical device, and depending on it's resistance (measured in ohms), a certain amount of current flows as a result. In audio terms, your amplifier puts a voltage across your speaker, and a current flows through, the amount of current depends on the voltage and the resistance of the speaker.

It just follows a simple equation: V=IR or I=V/R (I=current, V=voltage, R=resistance)
Another basic electrical equation is P=IV (P=power)

Using these together, you can get an equation like P=V^2/R, or P=I^2R, or many others

So say you have an amp that puts out 1000x1@4ohm. Using P=V^2/R, you can see that it will put out 63V at it's terminals. Using I=V/R you can see that 16amps of current will flow through your speaker as well. This is all fine and dandy, and not of much use, but the importance comes when you start dropping the resistance. A 1000x1@4ohm amp will always put out 63V at it's terminals when maxed out. With a 4ohm sub you get the 16amps flowing through that the amp was designed to handle and everyone is happy. If you were to attach a 2ohm sub to it you would then get 32amps flowing through, if you attached a 1ohm sub you would get 63amps, and so on. This extra current that the amp was not designed to handle can do some very bad things, such as melt pieces of the amp, explode capacitors, etc. If you're lucky the amp will detect the problem and shut itself off before any damage occurs. If you aren't lucky you'll have to buy a new amp.

Basically, a lower resistance means more current for a given voltage. If the amp can handle this extra current then it's fine, if not then you're screwed. This is why amps are rated down to a certain ohm load, because any lower than that and too much current will flow and the amp will burn itself up.