Your profile lets other people know things you want them to know about you. Some people are more private, and I totally get that. You don't have to enter profile information if you don't want to, and you don't even have to use your real or full name.
If you do want to enter profile info, that can be done at the User Control Panel, next to your name at the top of the site. Then click the Profile tab, and enter as much info as you want. Some of your profile information shows up to the right of each of your posts in the list.
You can also enter a signature in this area. A signature shows up at the bottom of your post.