Users on Guns Lot are able to have friends. By having friends you are able to view their recent activity and get notifications on new things they do.
A subscriber is somebody that adds you to their friends, but you don't add them. Because of this, they are the only one that will receive updates from you.
A friend is somebody that adds you as a friend, and you also add them as a friend. Both people will receive updates from each other.
A mutual friend is a friend that somebody has in common. So if you are browsing somebody's user page, anybody that they are friends with that you are also friends with will show as a mutual friend.