The short answer is that a VPN tunnel is an encrypted connection between you and your VPN.

What is VPN ?

It makes it so neither your ISP nor the sites you visit can see what you’re doing unless they crack the encryption—which in practical terms is impossible.

A VPN, or virtual private network, prevents some of this tracking in two ways: first by rerouting your connection and then by encrypting it.

When thinking about VPN tunnels, one handy way to visualize them is like you’re driving a car. When you’re driving on the open road, anybody can see you.

The moment you drive into a tunnel, it becomes a lot harder. To take the analogy a little further,

The upshot is that a VPN tunnel is a good thing to have, though it’s far from perfect. The biggest downside is that it will slow down your connection.