The Tunnel Entrance is an Alien structure, created by the Gorge, which is similar to the Marine's Phase Gate in function.
For an initial outlay of personal resources, the Gorge can dig a tunnel in the ground with an entrance in one location on the map, and an exit in the other, which can be used to travel quickly in a direct line across long distances. Tunnels are useful for helping your team cross the map rapidly.
Although travel through this tunnel is not instantaneous (unlike Phase Gates), it is still very quick, as the tunnel is like a pocket dimension- shorter than the actual distance between the entrance and exit.
Marines can enter the tunnel themselves if your team is not careful about guarding the entrance.