When you visit an encrypted website, Safari checks the site’s certificate and warns you if the certificate is expired or illegitimate. Now with iOS 11.3 and macOS 10.13.4, Safari also checks if a website that asks you to enter password or credit card information is not secure because it's unencrypted.
If Safari detects that the website is not secure, do not enter your information.
If an unencrypted website has requested your password or credit card information, Safari warns you that the page you are on isn't secure.
If you tap or click into the form to sign in or enter information, you'll see a more prominent warning in the Smart Search field.
Never provide your password or credit card numbers to sites with this warning. If you need to use the site, contact the site owner or administrator and let them know that their site isn't secure.