What is grapewine in communication..?

To hear something through the grapevine is to learn of something informally and unofficially by means of gossip and rumor.

The usual implication is that the information was passed person to person by word of mouth, perhaps in a confidential manner among friends or colleagues. It can also imply an overheard conversation or anonymous sources of information. For instance "I heard through the grapevine that Brad was getting fired."