It means that they enjoyed your post and feel it's worthy of being given a rep. For every 100 rep points gotten, you gain a power. If someone reps you, you gain as many points as they have in rep powers. For example, if wwjd_kilden repped you, you would gain 98 points, because she has 98 powers..
And you can give rep to others, and they will get as many points that you have in powers. Currently, you have no rep powers, so your rep comment would not raise anyone's powers up. Once you reach 50 posts, you get 1 power. It increases after that, and each increase is different for everyone. And for each year that you are with the site, you gain 1 rep comment/power.