HTTP delivered over an encrypted connection, helping protect information exchanged between a browser and a website.
HTTPS is HTTP carried over an encrypted connection using TLS. It helps protect data while it travels between a client, such as a browser, and the server. A properly established connection provides protection against others reading or altering that traffic in transit.
For a business website, this matters on ordinary pages as well as on forms and checkout screens. Visitors should not need to work out which parts of a site deserve connection protection. The certificate and server configuration also need to remain valid as domains, hosting and third-party services change.
An encrypted connection does not certify the honesty of a business or the quality of its products. It also does not replace secure application code, access controls or careful handling of customer information. HTTPS addresses one important layer of protection; it should not be described as proof that a website is safe in every respect.
Takeaway: Use a valid HTTPS setup throughout your public website and test important forms after hosting or domain changes. Treat certificate problems as operational issues, not just an SEO setting.
Example
A customer opens a quote form using an HTTPS address, but a recent domain change leaves the certificate invalid for that hostname. The page may look normal to the owner in another browser session while a new visitor receives a warning. Checking the exact customer-facing URL catches a problem that reviewing the page design would miss.
What to check
Ask whoever manages the site to verify certificate renewal, the preferred HTTPS hostname and redirects from old HTTP addresses. Test the actual customer journey rather than assume installing a certificate finished the migration.
For more detail, see MDN HTTPS reference.