Gatekeeper is the macOS security feature that verifies downloaded Mac apps before allowing them to run.
Gatekeeper checks whether software has an acceptable code-signing identity and, on supported macOS versions, whether Apple has notarized it. These checks are intended to reduce the chance that a user will accidentally launch malware downloaded from the internet. When an app fails the relevant checks, macOS can warn the user or block it from opening.
The feature was introduced as part of Apple’s broader effort to make software distribution safer beyond the Mac App Store. Gatekeeper builds on File Quarantine, which records that an app came from an external source, such as a web browser or email client. That quarantine information helps macOS decide when additional checks should be performed.
Gatekeeper is not the same as FileVault, which encrypts storage, or Keychain, which stores credentials and cryptographic secrets. Spotlight is a search system. Gatekeeper also is not an absolute malware guarantee: users can deliberately override warnings, and protection depends partly on quarantine metadata, signatures, and Apple’s security services.