HTTPS (Hypertext Transfer Protocol Secure) is a communications protocol used on the Internet that has a layer of security added. It is a combination of the standard HTTP protocol, and a security protocol called SSL/TLS.
HTTPS is important because standard HTTP sends data over the Internet in plain text making it easy to intercept. The added security layer with HTTPS encrypts information traveling over the internet to prevent wire tapping, stolen credit card numbers, and other interceptions.
All Cloudflare customers on a Pro plan or higher have HTTPS enabled for their website by default.
Related: Information about Cloudflare's SSL