Potential activity points are earned every 2 weeks that you are active. That means, for every 2 week period, you have to post at least one post to get 14 activity points. Your post has to be higher or equal to the potential activity for it to show.
Just link to some previous threads that explain it. No need to repeat oneself ad nauseum due to people not wanting to search '1 sentence' in Google. This should be adequate:
Activity & new membergroup limits:
https://bitcointalk.org/index.php?topic=237597.0Newbies - Read before posting:
https://bitcointalk.org/index.php?topic=1689727 [1]
[1] - You can find a small description near the end of the post. However, I highly recommend that you read it all.