A virtual private network (VPN) is a tool that allows internet users to hide their identity (with some limitations). It creates a proxied IP address to mask their true identity. It can make it look like they’re in another country when they actually aren’t.
While this can be a good thing for privacy advocates, it can also be used maliciously for people to try and cheat at contests. Therefore, SweepWidget disallows the use the VPN’s by default. You can, however, allow VPN’s if you prefer.