From the sticky at the top of this forum:
The activity number is determined in this way:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
Activity is updated every 30 minutes.
Maybe this line could be reworded to be a bit more clear. Not everyone on this forum understands programming.
In English:
number of 2 week periods posted in multiplied by 14
OR
number of posts
the lower number is your activity score