Every post and every comment has a karma number - on the post, it's the number in the circle next to the author's name; on the comment, it's the number between the date and the [-] thing that collapses the thread. "Vote up" adds one to this number, and "Vote down" subtracts one.
Some research says that lurkers make up over 90% of online groups. I suspect that Less Wrong has an even higher percentage of lurkers than other online communities.
Please post a comment in this thread saying "Hi." You can say more if you want, but just posting "Hi" is good for a guaranteed free point of karma.
Also see the introduction thread.