Secure Sockets Layer(SSL) is technology that allows secure(encrypted) communication from one computer to another.
It works by each computer having something known as a certificate, which is signed by a Certificate Authority(CA).
The CA ensures that the certificate has been validated and that the computer purchasing the certificate is in fact the computer and not someone else.
Once the certificate is signed it is then split into a public certificate and a private certificate. The public certificate can be used to encrypt something and this can only be decrypted by the private certificate.
If you are browsing to a website that is using SSL, the website will give you a copy of it’s public certificate. Your browser will then check against a list to ensure that the certificate is signed by a trusted CA.
Once validated, your browser will then use the public certificate to encrypt the data being sent to the website. Once the website decrypts this a secure channel of communication is established and the rest of the browsing session is secured.