HTTP Strict Transport Security (HSTS) is a web security policy mechanism that helps protect websites from certain types of attacks, such as man-in-the-middle attacks. When a website implements HSTS, it instructs browsers to only connect to it using a secure connection, specifically HTTPS, for a specified period. This prevents users from accidentally accessing the site over an insecure HTTP connection. When a browser receives an HSTS header from a website, it remembers this instruction for future visits. This means that even if a user tries to access the site using HTTP, the browser will automatically switch to HTTPS, ensuring a secure connection and enhancing overall web security.