What is post? You just made one
.... any of your reply to any thread or making your own thread is a post.
So one post daily will raise your activity (and rank) in max speed (14 every two weeks).
I think this person is talking about post that is equivalent to activity that will let you rank up. That is the post that this person talking about
A constructive post is:
Be on topic
Be helpful
Your reply to be constructive and to contribute to the discussion.
Since you are asking about activity etc, I think you are looking to join a signature campaign like 99% of all users do here. For that you need to wait at least three months until you become Full Member and then looking to join a signature. Meanwhile during this time follow what I said above, how posts should be.
This guy is right it should be help for your question and just to add up post are not counted as I see is the one that has 3-6 words lets say just create a post that has 2-3 sentence minimum.