Since Jailbreaking iOS 11 has became a very difficult task, users have started installing modified apps on their iDevices using Cydia Impactor. However, the issue with sideloaded apps is getting revoked. It is because they have a limited duration Apple developer license and once it is expired, users have to re-install or sideload these apps using Cydia Impactor.
Chances are high that if you own iPhone or iPad, you would be familiar with Cydia. It is sort of App Store for jailbroken iDevices where you will find lot of tweaks to customize your iDevice according to your liking. Emulators like iOSEmus are not allowed in App Store, so they found their way to Cydia.