HSTS stands for HTTP Strict Transport Security, it’s a web security policy mechanism that forces web browsers to interact with websites only via secure HTTPS connections (and never HTTP). This helps to prevent protocol downgrade attacks and cookie hijacking.
HSTS was originally created in response to a vulnerability that was introduced by Moxie Marlinspike in a 2009 BlackHat Federal talk titled “New Tricks for Defeating SSL in Practice.” The particular vulnerability that HSTS defends against is the one illustrated by Marlinspike’s SSLStrip tool.
Essentially the tool works by converting secure HTTPS connections back to unsecured HTTP ones. HSTS remedies this by communicating to the browser that an HTTPS connection should always be in place. HSTS can also help to prevent cookie-based login credentials from being stolen by common tools such as Firesheep.
Unfortunately, some HSTS settings can inadvertently cause browser errors.