Fly GPS is an application that 'tricks' your GPS into believing you're somewhere you're not. Likewise, you can make your device believe that you're in motion, which is the reason this application captured the attention of a lot of Pokemon GO players. If you perform a simple search, you can find lots of tutorials to help you set up Fly GPS to work with Pokemon GO.
Besides using it with certain games, Fly GPS also lets you do a lot of interesting things with your Android without having to move from wherever you are. For this reason, it can also be a useful and interesting app for developers who need to test apps.