Rightly put by pratik .. grapevine essentially means gossip.
As a management student, you should know how to make the grapevines work for you.
Sometimes grapevines give you indepth intentions of your staff.
Say, every manager has some " chamcha " who keeps the manager updated with whatever he hears..
He is also the person who can spread grapevines. Say a manager has to sack some non productive staff.. he asks the blue collered guy to start a grapevine... that way he comes to know about the reactions of every employee and take decisions accordingly.
Grapevines are also spread in cross companies to give the wrong information to their competitors and sense each others moves.