It is the Verge3 Map Editor. It is a development tool that you can use to create and edit custom maps for v3 to use. You can edit and place tiles, set up obstructions and events, and do other map-related stuff.

The Maped documentation has detailed information as to what everything does and how to get started, but note that you need the .NET runtime in order to run the program, but not to play any verge games you make with it.