A quick read on your site's security posture: HTTPS, security headers, and mixed content. Search engines and users both reward a site that is locked down.
The security checker reads a page and reports on the basics that protect visitors: HTTPS, HSTS, a Content Security Policy, clickjacking protection, mixed content, and the other headers a browser uses to decide what a page is allowed to do.
It runs in your browser against a single page. You get a plain read on what is set, what is missing, and what to fix first.
HTTPS and HSTS make sure traffic is encrypted and stays that way. A Content Security Policy limits which scripts and resources can load, which cuts down on injection attacks. X-Frame-Options stops other sites from framing your page to trick users (clickjacking).
Mixed content is when an HTTPS page still loads some assets over HTTP. Browsers may block those or warn users, and it weakens the lock icon. The checker calls out any of these that need attention.
Browsers flag insecure pages, and a warning bar pushes visitors away before they read a word. HTTPS is also a baseline ranking signal, so getting it right is table stakes.
Most of these headers take a few lines of server config. The checker tells you which ones are missing so you can hand a short list to whoever manages the site.