The "Resteem" button is the arrow in front of the "Reply" button. It's like the equivalent of sharing someone's post to your timeline, on Facebook.
When you like a post (and want to show it to your followers), you can resteem it so that it'll appear on your "blog." Moreover, posts can only be resteemed up to 1 week (I think?) after they were published; the button will then disappear.
it benefits the one who posted it right ?
Yes.