An SSL certificate allows a secure connection, also known as an encrypted connection between your website server and your visitor's web browser. When you do online banking and see that little tiny green lock in the address bar next to “www..?” This shows that the website has a secure connection to your browser at the time.
SSL Certificates encrypts sensitive information to ensure that any data is unreadable to everyone other than the destination server. When site visitors send data (such as credit card details or other personal information) over the internet it gets passed through multiple computers before reaching its destination server.
If this data isn’t protected by an SSL Certificate it could get stolen at any point during this chain.