"Rep Power" defines how many rep points you can give or take away from a user. So if you have a rep power of "0" and you give positive or negative rep, the net result is a change of zero either way. If you have a rep power of 1, when you give positive rep, it gives 1 point to the person's reputation, if you have a rep power of 3, you give 3 points, + or - (depending on if you give positive or negative rep).
Here's how that crazy rep power is calculated:
You get your first rep power at your 50th post, after that...
You get one rep power for every 365 days that you're registered
One power for every 1000 posts you make
One power for every 100 reputation points you receive
You can see rep points comparatively by going into the member list and clicking on "Reputation" to see who has the most reputation. While it can be superficial at times, I think it's a decent indication of how well a user is received and in many case, how informative or good their posts are...and I'm not just saying that because I have the most. I likely have more than others because I seem to give it out a little more often. While most people seem to say they have to spread the rep points around before they can give it to the same person again often, I've only encountered that a few times (I've been known to give rep to the same person within 72 hours). Of course, I have been locked out for giving out too much rep within a 24 hour period. The defaults are that you have to spread rep points around to X number of people before you can give it to the same person again, and you can only award so many points in a 24-hour time frame. I give it out to people that never give it back to me, but if someone gives it to me, I do try and keep an eye out for their posts to see what they're saying. There are so many good posts on this forum, I find it hard not to rep a lot of them. Some are longer than others (guilty

), but some are short, sweet and to the point while others show a distinct abundance of helpfulness, insight and caring…all kinds of posts are worthy of a rep point here and there.
The number of rep points also factors into the Rep Point phrases (as seen by holding your pointer over the number of green squares to the top right of a user's post), the listing below shows the default # of points and the default phrase:
-99999.....User is infamous around these parts
-50.....User can only hope to improve
-10.....User has a little shameless behavior in the past
0.....User is an unknown quantity at this point
10.....User is on a distinguished road
50.....User will become famous soon enough
150.....User has a spectacular aura about
250.....User is a jewel in the rough
350.....User is just really nice
450.....User is a glorious beacon of light
550.....User is a name known to all
650.....User is a splendid one to behold
1000.....User has much to be proud of
1500.....User has a brilliant future
2000.....User has a reputation beyond repute