impendence refers to ohms. the majority of full range car speaker's impendence is 4 ohm. check out http://www.bcae1.com . you should be able to find the answer to almost any other question you have there as well. anything else, let us know.
edit: impedence is covered under #43 if you scroll down on the right
There isn't a good or bad impedance rating. Its more a matter of what your trying to do, and matching the speakers ohm load to the amp.
An example is running 2 subs to a amp that can only run 4 ohms bridged. Since you have 2 subs you need their final ohm load to be 4 ohms so you can get the most out of the amp. Since there are two subs (depending on how you wire them) you will multiply or divide the restince to get the final load.
So you would want two 2 ohm speakers (wired in series to get a 4 ohm load) or two 8 ohm speakers (wired parallel to get a 4 ohm load).