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you pick a post and click on the resteem-button.
The resteem-button is on the left side of the Reply - "Button" (under the post).
It looks like an arrow.

Thank you, i did it on this reply. And so, what does resteem accomplish?

It's basically the same as sharing a post.

Ok, so the people that follow me will see that post on their feed?

Thats right, and if they decide to check it out, from your Re-Steam link, you could get a small payout as well.

It shares the post on your blog.