Posts means the number of posts you've made (till date).
Activity simply means you get 1 for each of your posts that you make during a period of 14 days, limiting it to 14 for 2 weeks. That doesn't mean that if you don't make 14 posts, you will not get the remaining ones in future.
In fact, even if you make at least 1 post (or 2 or whatever number but not exactly 14 or more) during a specific 14 days period before/after a cycle ended, you're entitled to get the remaining activity during the next cycle when you complete the remaining posts.
For e.g.; If you make 2 posts during a 2 week period, you'll receive 2 activity points. Though, during the next 2 week period, you can get your remaining 12 activity once you complete 12 more posts. Not just that, but as you've posted during another posting period, you're entitled to get 14 + 14 = 28 activity for the total 4 weeks. But remember, you need to make at least 1 post during a 14 days period to receive the remaining 13 activity any time later once you complete your remaining number in posts.
And for more info on merits and new rank requirements (as I believe you should know them as well):
https://bitcointalk.org/index.php?topic=2818350.0