HTTP Strict Transport Security, or HSTS, is a web security policy that helps protect websites from certain types of attacks, such as man-in-the-middle attacks. When a website uses HSTS, it tells browsers to only connect to it using a secure connection, known as HTTPS, instead of the less secure HTTP. This ensures that all data exchanged between the user and the website is encrypted. When a browser receives an HSTS header from a website, it remembers this instruction for a specified period. During this time, if a user tries to access the site using an insecure connection, the browser will automatically redirect them to the secure version. This enhances user security and builds trust in the website.