App is the short expression for "application". It is a software program you can use while online or on mobile devices.
It usually has a single and simple purpose. In the context of OnSign TV, it is a small pre-designed program which allows you to display dynamic content – such as time, weather, news from websites, videos from YouTube, and much more.
The picture below shows the App library, pausing on the Sports tab. All categories are listed on the left bar menu, each of them containing a variety of apps with different functions and styles. All are customizable.