A mash-up can be seen as a composite app that is combining reusable data, presentation, and new logic. It is often seen as web solution, but this approach can be used for native app development as well. Data is everywhere. The government and various organizations have made their data publicly available through APIs. Mash-up solutions do not need to worry about the content in particular, but more about the presentation. They may occur as enterprise, data-oriented, or consumer mash-ups.
The app may gather data from multiple sources, combine and enrich them, and then present them in an app. An example of that could be as simple as producing infographics from the provided data. Another example is getting photos from Flickr and presenting them on a Google map. There are plenty of other and more sophisticated solutions that you can think of. A mash-up can be a great contribution...