If networks are two geographically separate networks, which want to communicate with each other, they may deploy a dedicated line between them or they have to pass their data through intermediate networks.
Tunneling is a mechanism by which two or more same networks communicate with each other, bypassing intermediate networking complexities. Tunneling is configured at both ends.
When the data enters from one end of the Tunnel, it is tagged. This tagged data is then routed inside the intermediate or transit network to reach the other end of the Tunnel. When data exists in the Tunnel its tag is removed and delivered to the other part of the network.
Both ends seem as if they are directly connected and tagging makes data travel through the transit network without any modifications.
