There is two way to use a VPN :
- Access to a private network from outside : If you want to access to a network behind a router, you can use a VPN to create an secured connection and access this network from outside
- Hide your IP and encrypted your connection : You can also buy this service from a VPN provider, it will encrypt all connections between your computer and the provider server, and show the server IP instead of your IP.
But for this usage, make sure the VPN provider do not keep any log, and don't use servers from companies under the Patriot act. For example HideMyAss use AWS servers to run his VPN service, so all data can be used by the NSA or another US agency if they ask for.