what is a VPN? and what the function of a VPN?

VPN (Virtual Private Network) is a tool that helps our security when surfing on the internet world. We can secure your computer from hacker and cracker. VPN make your IP anonymouse when you are surf on the internet that you do not know secure or not.VPN is an emulation of a
private Wide Area Network(WAN) using shared or public IP facilities, such as the Internet or private IP backbones.

Secure VPNs use cryptographic tunneling protocols to provide confidentiality by blocking intercepts and packet sniffing, allowing sender authentication to block identity spoofing, and provide message integrity by preventing message alteration.

VPN is a virtual connection that is private because basically it is not physically exist only in the form of a virtual network and why it is called private? because this network is a network of private nature which not everyone can access it. VPN Connecting the Personal Computer with a public network or the Internet but are private, because it is private so not everyone can be connected to this network and access it. Therefore, data security needed

The concept is basically the work VPN VPN Requires a server that berfungsing as a liaison between the PC. If described something like this

Internet <-> VPN Server <-> VPN Client <-> Client

when used to connect 2 computers in private with the Internet network so like this:
Computer 1 <-> VPN Clinet <-> Internet <-> VPN Server <-> VPN Client <-> Computer 2

also you can check on this other post
11 best Free VPN