HTTP Strict Transport Security (HSTS) is a web security policy mechanism that helps protect websites against protocol downgrade attacks and cookie hijacking. It allows web servers to declare that web browsers (or other complying user agents) should only interact with it using secure HTTPS connections and never via the insecure HTTP protocol.
HTTP Strict Transport Security (HSTS) preload (also called HSTS preloading) ensures that browsers always connect to a website via HTTPS and never via HTTP. It preloads a domain into a list that a browser checks before loading a domain, enforcing HTTPS by default from the very first connection.