HSTS is a tool that tells browsers to only use SSL for all future requests to a website. It strengthens SSL by preventing many common attacks against it (such as protocol attack and cookie hijacking).
Enabling HSTS is simple
Step 1 - Access and edit your .htaccess file. Click here for a guide if you're unsure how to do that.
Step 2 - Add this line to the top of the file:
Header set Strict-Transport-Security "max-age=31536000" env=HTTPS
Step 3 - Hit Save Changes on the top right of the screen