A secure socket layer (SSL) certificate is a digital certificate that enables encrypted communication between a web server and a client, typically a web browser. SSL certificates authenticate the server's identity, ensuring that sensitive data exchanges, like credit card numbers and personal information, occur securely. When a website uses SSL, it displays "https://" in the URL, indicating an encrypted connection. SSL certificates are crucial for establishing trust with users, as they help protect against data interception and man-in-the-middle attacks, making them essential for e-commerce sites and any platform that handles private information.