Tribal Knowledge - as the name suggests, it is the information known within the tribe but not outside of it. In a corporate set up, it is the information that is known to a certain group of people and is NOT known to the others in the organization. Tribal knowledge is either gained over time or is verbally passed on from one person to another and might be important to the production of a product or performance of a service. It is usually not documented and hence there is a risk of this knowledge getting lost if the people who possess it choose to move out of the organization.
An application oriented question on the topic along with responses can be seen below. The best answer was provided by Venugopal R on 29th October 2017.