What is HSTS?

HTTP Strict Transport Security

HTTP Strict Transport Security (HSTS) is a web server directive that informs user agents and web browsers how to handle its connection through a response header sent at the very beginning and back to the browser.

This sets the Strict-Transport-Security policy field parameter. It forces those connections over HTTPS encryption, disregarding any script's call to load any resource in that domain over HTTP. HSTS is but one arrow in a bundled sheaf of security settings for your web server or your web hosting service.