Gatekeeper

Gatekeeper is the macOS security layer that checks software the first time it runs. Depending on how the app was distributed and signed, the user sees anything from nothing at all (the good case) to ""App" can't be opened because Apple cannot check it for malicious software" with no obvious way forward.

What Gatekeeper checks: a valid code signature from an identified developer, notarization by Apple, and a quarantine flag browsers attach to downloads. Apps that pass all three open without any dialog; apps that fail get warnings that most users treat as a hard stop.

A related quirk: macOS App Translocation runs quarantined apps from a randomized read-only location until the user moves them to Applications - which breaks self-updating for apps run straight from the Downloads folder. It's one of several reasons Mac apps ship as DMG images with a drag-to-Applications step.

Deskifier's Mac builds are signed, notarized and stapled, so Gatekeeper opens them without complaint.