Secure Sockets Layer (SSL) is a standard security protocol used to establish encrypted links between a web server and a browser, ensuring that all data passed between them remains private and secure. SSL is critical for protecting sensitive information, such as credit card numbers and personal details, during online transactions. Websites that use SSL are identified by the "https://" prefix and a padlock icon in the browser address bar. SSL has since been replaced by Transport Layer Security (TLS), but the term SSL is still commonly used. SSL certificates are required for websites to provide a secure browsing experience for users.