HTTP Strict Transport Security is a security feature that lets a web site tell browsers that it should only be communicated with using HTTPS, instead of using HTTP. The first time your site is accessed using HTTPS and it returns the Strict-Transport-Security header, the browser records this information, so that future attempts to load the site using HTTP will automatically use HTTPS instead.
When the expiration time specified by the Strict-Transport-Security header elapses, the next attempt to load the site via HTTP will proceed as normal instead of automatically using HTTPS.
Whenever the Strict-Transport-Security header is delivered to the browser, it will update the expiration time for that site, so sites can refresh this information and prevent the timeout from expiring.
Refer - https://developer.mozilla.org/en/Security/HTTP_Strict_Transport_Security