I'm not 100% sure if I understand the rep system but I'll take a shot at explaining it in this post.
So if I understand Reputation correctly, when you first begin posting on the site you begin with 5 reputation points. You are not able to "give rep" until you have at least 15 Rep Points AND
100 posts. This is to prevent people from making new accounts or docking rep/pumping rep to certain users. Before you reach 15 points + 100 posts, you are able to give rep, but the recipient does not receive any rep points. (some appreciate being acknowledged for their good posts though so don't let that stop you from giving someone rep if you liked their post).
From there you then are given a rep power of "1", and that rep power goes up under any of the following circumstances:
1. Every 100 rep points that are given to you
2. Every 1000 posts that you get to
3. Every 1 year anniversary of you first joining the site.
With regards to the little green bars, you get one of those dark green bars for each 100 rep points that you receive...until a certain point when you get those bright green bars which are given out for every 200 rep points you receive after you pass 500 rep points or something. (this part I don't quite understand). A mod can feel free to edit this post to include whatever I forgot or don't understand well enough...or simply delete it if its wrong lol.
Edit: this thread
will help you get a better grasp of the rep system. Just realized everything that I said in this post is on there...might have even been that thread that I read that explained it to me lol.