SNAP is the common software platform for the three Sentinel Toolboxes which are developed by the European Space Agency (ESA) for the scientific exploitation of the Sentinel-1, Sentinel-2 and Sentinel-3 missions.
This is the source code distribution of the SNAP Desktop GUI which is developed on top of the SNAP Engine (snap-engine) and is based on the NetBeans Platform. For further information read the readme.md in the SNAP Engine project
The project page of SNAP and the sentinel toolboxes can be found at http://step.esa.int. There you will find tutorials about the usage of the application, a forum where you can ask questions and lots of other interesting things.