An SSL certificate is a digital certificate used to ensure the security of data exchanged between a web server and a browser. It is an important means of ensuring user privacy and security, as it provides an encrypted connection between a user's browser and the web server on which a website is hosted.

When a website has an SSL certificate, the website's URL is preceded by "https" instead of "http". This indicates that the website is secure and that data exchanged between the browser and web server is encrypted and cannot be intercepted by third parties. An SSL certificate is typically issued by a certificate authority (CA) and can be validated by viewing the certificate details in the browser.