A piece of software that sits on your device and hides your IP address and user credentials from “snooping” network users. Of particular value when connecting to an unsecured public network (eg a cafe). Normally unnecessary on a home network when connecting to you router, but it obviously can provide additional security and hides your device completely from the internet.
Here is a link that explains how they work, and another one from the provider I use (NordVPN).
One additional use that they have is that because you can choose the geographical location of the VPN server you wish to connect through, you can logically (or virtually) place yourself in a different country, thus enabling you to appear to be in the UK – when abroad (so that you can watch UK TV); or alternatively appear to be in a different country from home (when assessing alternative cost options for say a holiday).