Strict-Transport-Security (STS) is a web security header that helps protect websites from certain types of attacks, such as man-in-the-middle attacks. When a website uses STS, it tells browsers to only connect to it using HTTPS, ensuring that all data exchanged is encrypted and secure. When a browser receives the STS header, it remembers the instruction for a specified period. During this time, even if a user tries to access the site using HTTP, the browser will automatically switch to HTTPS. This enhances user security and helps prevent data interception.