The little up and down arrows to the left of each post let you upvote or downvote a post. (If you're below the karma threshold, you won't see the down arrow.)
Nobody can see other people's scores, so it's not a good way to indicate agreement. In fact, it's unclear as to what the purpose of voting is at all.
The purpose of voting is to sort comments on the page, so that you can save time by only reading the first (theoretically, the "best") N comments in a thread.
Voting affects the order of comments: comments with a higher score (among other factors) will appear nearer the top.
Sweet, so in a couple more years I'll finally be able to see that down arrow! :)
It's changed a few times over the years, so I wouldn't get your hopes up. I've lost and gained it before. I think it was around 100 when I joined, then it went up to 250 or so, now it's apparently 500.
what's the magical threshold? I've been on here for years and I'm only at 31