Think of impedance as pipes going from your amp to your subs and your amp as a water pump. Every amp is willing to *try* to pump as much water (power) to your subs as the piping will allow it to. The pipe's size is the impedance, which is decided by the subs wiring, the lower the impedance, the bigger the pipe is, the higher the impedance the smaller the pipe is. Based on simple physics, we see that the lower the impedance, the bigger the pipe is, and the more water (power) the amp will try to deliver. I say *try* to deliver because like any piece of machinery, every amp has its limits on how much power it can deliver before it fries its insides and blows. This is where the terminoligy of, for example, "1 ohm stable amp" or "2 ohm stable amp" comes from. Thats the lowest impedance that those amps can handle before the overwork themselves and blow.
Impedance is one of the most confusing parts of car audio, but also one of the most important things to learn. It really relates to all electric stuff in general so its a good life skill to be versed in it.