It happens out of the blue. I got some today. Normally, you +1 via a button, but on these boards it appears to be code disabled. basically you get it for knowledge, helping peeps and being nice to the community. That is how I see anyways.
There are really two ways to acknowledge or give someone credit ("+1" someone) in our current system. The quick & easy one is to hit the "Thanks" button at the bottom left of the post. This is a quick way to acknowledge a post, to agree with it, or to say "good job" publicly, without ending with threads full of "what he said!" It's really pretty much the same as "Liking" something on Facebook or some other forums.
The other is the Reputation System. People have already posted links to the official stuff... so here's an informal explanation. Basically, the rep system gives an informal assessment of a poster. You can give positive or negative rep for a post, based on what you think of it. We encourage, though we don't require, that you sign rep you give and put some sort of comment in about why you're giving it. For example -- sometimes all I say is "good post - jks9199". (I personally haven't ever felt the need to give negative rep -- but reserve the right to do so in the future.) Other times, I might make a comment that I don't want to make publicly for one reason or another ("you've got a good point; too bad that they're so locked in their view they're not going to see it.") Based on your own reputation point balance, you can dish out more or fewer points when you rep someone. Some folks don't "play" in the system, too... their rep indicator is gray. They can still use the system to comment and send rep -- but there are no points involved when they do it. The idea behind the rep system is that people who write better posts get more rep points, hence more green bars or stars, and gives a clue to others that they've written fair number of solid posts -- or that they've put their figurative foot in their mouth an awful lot, if it's deep into the red.