>>106025162
Basically instead of sending your network requests straight to a website, you send the request to the VPN's server, who then forwards your request to whatever site you were trying to access and gives the result back to you. Meaning the website only sees the request coming from the VPN's server, not your IP. This has the added benefit of spoofing your location if the VPN server is in another country, hence it can circumvent geoblocks.
You can turn VPNs on and off freely - all the big ones will have an app you install that handles everything for you, you just turn it on or off, as well as picking the server location. Yes 4chan pass lets you post while using a VPN.
Self hosting a net-exposed server with a VPN is a bit more complicated, not all of them support port forwarding so look out for that.