Breaking down its abbreviation, a VPN stands for 'virtual private network'. Its primary function is to better improve security and privacy while online through any number of your personal devices.
It does this with its hundreds, sometimes thousands, of servers. Connect to one of these and your IP address will be changed, meaning you won't be tied to an IP your Internet Service Provider or government can track you on. It also utilizes encryption and other security features to ensure your personal information is kept secure as it travels from A to B.
By changing your IP address, this subsequently makes you appear as if you're browsing elsewhere. As well as boasting better online anonymity, this also means you can spoof your device and certain websites with geo-restrictions into thinking you're elsewhere, gaining access to overseas content and so on.