What does it mean to have a "Best Friend"?

Well I mean all of my close friends have like one “Best Friend” they tell everything to, and iv’e had some “best friends” but like I don’t really have one friend I can tell EVERYTHING to. When I hang out with my two closest friends I feel awkward because they are best friends and I’m kind of like the tag along. Like this weekend I was there for a day and the other to girls stayed together for 4 nights. I wonder if its weird I don’t have one best friend, and I was just wondering how you would define best friend, and how necessary it is to have one.