Decss, also known as DeCSS, is a computer program that can unlock movies that are protected by something called digital rights management (DRM).
DRM is like a lock on a movie or other digital content that stops people from copying or distributing it without permission from the people who made it. It's a way for companies to protect their property and make sure they get paid for their work.
Decss is like a key that can unlock this lock and allow people to copy and distribute the movie without permission. It was created by a person who wanted to be able to watch movies on their own computer without having to use a special device, like a DVD player.
However, since Decss is used to break the lock put in place by DRM, it is considered illegal in many places and can get you in trouble with the law. It's important to respect the rights of the people who make movies and other digital content and only use them in ways that are legal and ethical.