In more detail
What the warning means, and how to clear it
When a website uses a secure connection, the information passing between the visitor and the site is encrypted, so it cannot easily be read or changed along the way. Browsers mark secure sites with a padlock and an address that starts with https. When a site does not use a secure connection, modern browsers warn visitors that it is Not Secure. For a customer deciding whether to trust you, that is an off-putting first impression.
The fix is an SSL certificate, sometimes called a TLS certificate. It is what allows your site to use https. Many hosts now provide certificates free and can switch them on for you. If yours does not, it is worth asking why.
Installing a certificate is only part of the job. Every page should load securely, so anyone who types or follows an old http address should be redirected automatically to the https version. Images, scripts and other files on the page also need to load over https. If some still use the old address, browsers may show a warning even though you have a certificate, which is known as mixed content.
Certificates expire. If one lapses, visitors can see a full page warning that is even more alarming than the Not Secure label. Setting the certificate to renew automatically, and checking that renewal is working, prevents this from catching you out.
A secure connection also matters beyond the warning. Browsers and search engines treat https as the expected standard, and any form on your site, from a contact form to a checkout, should only ever send information securely.
The test from the video takes seconds. Open your site and look next to the address. If you see a padlock on every page, you are covered. If you see Not Secure, it is time to fix it.
