It's the main image of your profile, the one that is shown when someone is viewing the profiles listings, and the one in the heading of your public profile
You can select one of the photos you've uploaded in your photo galleries to make it also your "profile photo"
To select your profile photo, go to the "photos" option in your account menu.
If you prefer to use your own logo instead of one photo from your galleries, you can use the option you'll find in the "edit profile" section in your account. ¿What's the logo?