What does friend request sent on Facebook mean

It means you have sent a friend request to someone.

It means someone thinks they know you and are asking you to accept them as a friend on Facebook. Depending on your security setup, only friends can see things that list on your profile or place on your wall.

Really nothing except they want to connect or contact you, they may know you, have some common interest, or may just want to join some game on Facebook like farmville