I could tell you, but then I'd have to kill you.
No, seriously, it's an arcane formula based on longevity, post-count, and "rep" awarded by fellow members. To be honest, I don't think anyone knows exactly how it's calculated/weighted (kind of like the electoral college
). But essentially it rewards participation and quality content.
Over time, you end up with some combination of green squares and a "rep" number. Again, I'm not sure exactly what the relation is between the two, and frankly it's not all that important either.
You can reward another member for an especially helpful post, by giving them extra "rep". To do that, you click on the little "scales of justice" icon in the bar above their post. You can leave a comment too, if you'd like, or just a quick thank you. But you can't consistently give just a few people rep power. If you try to, you will be told by a pop-up dialogue box that you need to "spread some rep" around before giving more to the same person.
We used to have something called "negative rep", which allowed folks to ding other members for posts they didn't like. Sometimes it was justified, but we ended up getting rid of negative rep, because some of the grumpier members were habitually abusing it. But you can still still see where some members accumulated quite a bit of negative rep -- instead of a green square by their rep power they have a red square.
Probably more than you ever wanted to know about rep power. But a final comment: When you award rep to another member, it is done anonymously, unless you leave a note and sign it (which is a nice way to do it, so they know that you appreciated their help). BUT, moderators can always see who awarded the rep or left the comment, so it isn't untraceable.