The state of being where you have requested someone (me perhaps?) as a friend and she is not so sure she wants to add you (doesn't know you, haven't seen you in 15 years, etc). She does not want you to think she has rejected you because we all have tender/delicate feelings, so she just leaves you lingering there in Facebook Limbo - not accepted, not denied. That way, you aren't rejected and you can't keep requesting her - forcing her to reject you over and over again - not fun for any party involved.
C'mon - you know you do it.