Emulator is a software, primarily used for development, that allows running android apps on a computer. Most of the time Android apps are not meant to be run on emulators by the users.
Detecting when an app runs on an emulator is a key RASP feature. When an app runs on an emulator it is a sign of a possible attack or a reverse engineering attempt.
Malwarelytics for Android is able to detect when the app runs on an emulator and terminate the app if desired by the configuration.