Rep is like a system of points for being helpful.
Every 100 rep points you receive, will result in 1 reputation power.

What you see at the top of your posts are reputation power.
Another way to gain 1 reputation power is to make 25 posts on the forum.
Every 25 posts result in a rep power added to you rep.

Don't let this number get to your head because spamming and being unhelpful can result in people -reping you which will cause your rep power to go down.

When someone posts there is like a green thing in the bottom right corner of their post, and click click it, but atm you can only give 1 rep, but when your goes up you can give how ever much rep you wanna give.

Thanks alot,i +rep to whoever responded

Welcome to the site,

You give rep you click the green flag next to a post, you can either give +Rep by agreeing or -Rep by disagreeing and you can only give the amount of rep that you have e.g. I have 49 Rep so I can give someone 49 Rep.

To earn rep power you have to make 25 posts (26 for it too register) or you have to be given 100 rep from other people.

And you need 100 rep power per little green square e.g. if you have 22 rep power you will have two green squares and so on. But you are a quite far from that.