You give rep to a specific post. So if you see a post you like (most of the time the post will contain a map, but maybe someone left a helpful comment) you can then give rep to the owner of that post.
To give rep, you click the scale icon just below their username. You can give a reason for the rep (or a comment as it's called, but only the person you repped can see the comment) and then click the "Add to reputation button".
Now, you might want to practice... *hint* *nudge*