In each 14 days period, you can get a maximum* of 14 activity points. When you make posts less than 14 in those 14 days periods, remaining points will be accumulated which you can get by posting later.

To calculate potential activity, count all the posts you made in 14 activity periods and count all the 14 activity periods in which you posted. Then, potential_activity = (14 * no._of_periods_you_posted) - no._of_posts_you_made_in_those_periods. If you get a positive number, it is your potential activity and if you get a negative number or zero, you don't have any potential activity. You can see a list of activity timings here -- https://bitcointalk.org/index.php?topic=582736.0.

* Except if you have potential activity.