Gatekeeper is a security feature in macOS designed to protect users from malicious software. It works by verifying the source and integrity of apps downloaded from the internet before allowing them to run on a Mac. Gatekeeper checks whether the app is signed by a recognized developer and whether it has been tampered with. If an app does not meet these criteria, Gatekeeper blocks it from launching, helping to prevent malware and unauthorized software from compromising the system. This feature is a key part of Apple's layered security approach, giving users more control and confidence when installing new software.