Post shows the number of comments(posts under threads) you have contributed to the forum.

activity the same says maybe it's something related to "Activeness", literally how active you are and if you are making quality posts not spams. You can acquire activity through posting. So by raising your posts, your activity will increase. Your activity is counted within a period of 2 weeks which gives you a maximum of 14 activities or 14 days of being active in your account.