Im new here, just became member, what is the activity box for?

You can post in it just like here.
Just consider it your own personal bulletin board. Anyone who wants to see what you are up to can check it and reply.
Unfortunately anything you post will eventually disappear after a while.

Mine was empty except for one entry.
I just added another, but I really don't bother with it much since stuff doesn't stick.
The one item that was there disappeared when I added the new post.