A VPN tunnel (often simply referred to as a VPN, or virtual private network) is an encrypted connection between your computer or mobile device and the wider internet. Since your connection is encrypted, nobody along the VPN tunnel is able to intercept, monitor, or alter your communications.
All your data that is traveling in a VPN tunnel that nobody can look into. This protects your browsing, chatting, and all other traffic from the prying eyes of your internet service provider, your government, or people trying to monitor your activity over free public Wi-Fi.
A VPN tunnel not only protects you from data being intercepted, but it also hides your IP address, which can otherwise be used to identify you when you are browsing the web. Instead of your real location, the sites you visit will only see the location of the VPN server you are connected to.
This is how VPN tunnels can help you both circumvent censorship imposed on your local network and censorship being imposed by the local network of the site you are visiting.