Provisioning profile
The file tying together app ID, signing certificate and permitted devices — and the thing iOS developers have a famous love-hate relationship with.
The profile is baked into the app and tells the device that this particular app, signed by this particular certificate, is allowed to run. Development builds additionally list each device's UDID explicitly; a phone not on the list refuses to launch the app.
The combination of expiring certificates, profiles needing renewal and keys needing to be shared across a team is a classic source of build failures with distinctly unhelpful messages. Xcode's automatic signing and tools such as fastlane have eased it, but "code signing error" remains among the most recognisable failures in iOS development.