What is a network tunnel?
In one sentence: A connection the server makes outwards to a helper network, so visitors can reach it without any open doors on the server itself.
A little more
Normally a server opens a port and waits for visitors to connect in. With a tunnel, the server reaches out to a helper network and keeps that connection open; visitors connect to the helper, and their requests are passed back down the tunnel.
Because the server never accepts connections from outside, there's no open door for attackers to knock on. This site is reached that way.
It's a bit like… Instead of leaving your front door open for deliveries, you keep a phone line open to a trusted reception desk that passes parcels through a hatch.