See this thread:
https://bitcointalksearch.org/topic/activity-new-membergroup-limits-237597Basically it means you can earn up to 14 activity, every 14 day period, assuming your post count is greater than or equal to your potential activity.
For example, say you first join and make 30 posts in your first week, you would earn up to 14 points of activity. You would also have 16 posts counting toward potential activity waiting for the next period. Once the next period arrives, and you make at least one post, the entire 14 activity for the period will be credited, since you had 16 potential activity already waiting.
Sounds complicated, but basically it amounts to you can only earn activity at the rate of 1 activity per day over the long haul, also assuming you make enough postings. So in one year you could earn a potential 365 activity.
Most people will have more posts than activity. My thought is that activity was introduced as a way to keep people from spamming the forums with 100's of nonsense posts and becoming hero members in a day or two.