To secure the users of a website, the owner must obtain a digital certificate from a certificate authority or CA. With the help of the certificate, a secure connection is established between the user and the relevant service - between client and server.
When the client (for example a browser) contacts the server to establish a connection, the server sends a copy of its SSL certificate in return. The client checks that the certificate is valid and issued by an approved certificate issuer. In that case, the client gives a signal to the server, which establishes a secure communication channel between client and server.
Modern browsers have a field to the left of the address field that shows whether a website uses SSL. If it does, a padlock will appear. How it looks depends on the security level. If the website does not use SSL, the user is made aware that the connection is not secure, and the browser warns against exchanging sensitive information or blocks the page.