The small circle with the white V in it to the right of each post, photo and question allows you to do several different things, one of which is mark that post as a favorite.
Why would you want to mark something as favorite? To easily keep track of it! Any post, photo or question you mark as a favorite can be found in the My Profile section, after clicking the menu icon. You can also find it on your profile page, below your profile photo and to the right.