Checking the website’s address
When you open an HTTPS site, it sends your browser a certificate naming the web addresses it covers. Your browser checks that the address matches, the certificate has not expired, and a trusted certificate authority issued it. That organisation checks who controls the address.
People also call this an “SSL certificate”; TLS is the newer system used today.
Why warnings appear
If the certificate has expired, names a different address, or comes from an untrusted issuer, your browser warns you. A valid certificate helps protect your connection to that address; it does not vouch for the site’s content or business.