An SSL certificate is a digital certificate that authenticates the identity of a website and enables an encrypted connection. It ensures that data exchanged between a user's browser and the website remains private and secure. When a website has an SSL certificate, its URL begins with "https://" instead of "http://", indicating that it is secure. SSL stands for Secure Sockets Layer, a technology that protects sensitive information like passwords and credit card numbers during online transactions. By using an SSL certificate, websites can build trust with their users, as it assures them that their data is being handled safely.