An SSL certificate functions as a security protocol for web traffic by encrypting data exchanged between a user's browser and a server. This prevents eavesdropping and data tampering, ensuring that sensitive information, such as passwords and credit card details, remains safe. When a site has an SSL certificate, it is signified by "https://" in the URL, enhancing user trust and compliance with data protection regulations. Ultimately, SSL certificates contribute to a more secure internet experience.