WebRTC (web real-time communication) is a protocol that allows making audio and video calls from the browser.
However, WebRTC also allows websites to know your real internal and public IPs even if you use VPN (it's known as WebRTC IP leak).
Browsec can reliably protect you from WebRTC leaks. This option is turned off by default because it may affect the quality of audio/video calls. To turn WebRTC protection on, click the gear in Browsec and check 'Use Browsec for WebRTC connections':