WebRTC (or Web Real-Time Communication) is a technology that allows you to use real time voice and video communication within web pages.
While WebRTC is incredibly useful for communication, it can also be detrimental to your privacy and anonymity. WebRTC is able to bypass the browser proxy and obtain your real, private IP address from the system so even if your Windscribe extension is connected, WebRTC can still leak your real IP. However, please note that this is an issue if you're only using the browser-level proxy, not when you use the desktop VPN (read below for more info).
This is why the Windscribe extension gives you the option to disable WebRTC in your browser through the use of the WebRTC Slayer feature. You can access it in the Windscribe extension by opening the Privacy menu from the button in the top-right of the interface, and then switching on WebRTC Slayer. The toggle will appear as blue when this feature is enabled.