A configured tunnel is a point-to-point interface. It enables an IP packet to be encapsulated within an IP packet. Configuring a tunnel requires both a tunnel source and tunnel destination. See the tun(7M) man page and Solaris Tunneling Interfaces for IPv6 for more information.
A tunnel creates an apparent physical interface to IP. The physical link's integrity depends on the underlying security protocols. If you set up the security associations securely, then you can trust the tunnel. That is, packets that exit the tunnel originated from the peer that was specified in the tunnel destination. If this trust exists, you can use per-interface IP forwarding to create a virtual private network.