You can express that you like something by liking it. If you are logged in, you can click on "thumbs up" icon corresponding to the page, edit, or comment.

When should I like something?

You think it's high-quality: well written, informative, helpful, valuable, important, thought out, educational, etc...

You think the edit notably improved a page, and want to encourage more like it.

You want to see more pages like it.

You think the author used 3s appropriately.

DO NOT like a page because you think the claim the page makes is very likely true. Use voting instead.

DO NOT like a page to give your friends more karma.