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.
If you post once in each 14-day period, you will mark that period of activity with that single post and have a potential 14 activity points. You will only get that 14 activity points whenever you make enough 14 posts.
Why do I say it is potential point? You can make 13 other posts one month, two month later in order to fill that lack of 13 posts for 13 potential activity point in that 14 days.
If you notice, for users who currently have total posts exceed total activity points, after the activity recount of the forum, if they make only one post (to mark a new 14-day period), their activity will be updated with 14 more points (usually 1 hour after the post publishing time). Because their abundant posts in the past will be counted for the new 14-day period as well.