Rep points are a vbulletin (the forum software we use) system.

They are used so you can give "reputation" to a person who made a post if u find it informative/funny/whatever. You can also take reputation away if you dont like the post, find it offenseive e.t.c

So, the more points u get, the more points you give out if you "rep" someone else.

If basically a system of showing forum loyalty, and genuie posters, and helps you see who you can trust to be accurate on info and that kind of stuff