It's a bit confusing to newcomers, I get it.  I don't know if I can explain it any simpler than anyone else here has, but it'll make more sense as time goes on and you keep posting.

Basically as I understand it, when you first register you get 1 activity point for every day that you post up until an activity of 14.  After that, if you post at least once in every 2-week period, you'll get 14 activity points.  So if, at a minimum, you post once every two weeks from here on out, you'll increase your activity by 14 points--no more, no less--every two weeks when the activity period changes, usually on Tues-Wed, depending on your time zone.

Hope that was helpful and not more confusing.  I'd read the activity description when I was new, but it took more than a day for me to really understand how it works.  Then once I understood, it was simple.