Recommended Read: What is HTTPS

Hypertext Transfer Protocol Secure (HTTPS) is a combination of the Hypertext Transfer Protocol with the SSL/TLS protocol to provide encryption and secure identification of the server. HTTPS connections are often used for payment transactions on the World Wide Web and for sensitive transactions in corporate information systems

Recommended Books:

Introduction to Security, Eighth Edition  The Web Application Hacker's Handbook: Discovering and Exploiting Security Flaws  Web Protocols and Practice: HTTP/1.1, Networking Protocols, Caching, and Traffic Measurement  HTTP Essentials: Protocols for Secure, Scaleable Web Sites

Official Gmail Blog: Default https access for Gmail