Resteem means to share anyone's post to your friends. It's as like facebook post share or twitter re-tweet. Now benefit- yeah there are many benefit of resteem a post.Think you have only 400 followers and your friends has 400 followers. I you create a new post your post goes to only to your 400 followers. If your friend resteem your post your post goes over to more 400 peoples. That's the benefit. And you also get more upvote and more popular by resteem a post.