A mobile app i.e. a mobile application is an application developed for mobile phones and smartphones. In simple terms, a mobile app is a software programme for ones phone. These can be as simple as a basic flashlight application or a dictionary or something similar to a Google Maps navigation app.
There are mobile apps developed for entertainment like the Facebook and Twitter app (social media networks) or the Angry Birds mobile app (games). Mobile applications are basically a lighter version of computer applications meant for mobile devices/tablet devices, which cater to infotainment and entertainment.