OpenVPN is a VPN protocol and is considered to be one of the most widely used VPN software solutions. It is also an open source software which means that everyone can freely use it and modify it as needed.
OpenVPN uses a custom security protocol which relies on SSL for key exchange. OpenVPN can be used for various connectivity needs but its especially popular for anonymous and private access to Internet.
The ProtonVPN application uses OpenVPN as the default connection so that all your internet activity remains private and secure. Visit this page to see how you can change VPN protocols.