First things first: people are arguing over whether this is actually a map editor, but that's what the creator—Guadmaz—has called it. From what I can tell, this mod doesn't allow you to change the layout of the world in GTA V but does let you spawn objects like ramps and vehicles into that world.
The creator compares it to the Multi Theft Auto: San Andreas map editor, so if you've used that you should have some idea of how this will work. Here's the description:
The video uploaded to YouTube by GTA-Modding.com is from August and the mod has been updated since then, but it shows how you can fly around with freecam and spawn objects from a list. Go to 3:40 to watch the demonstrator dump a humpback whale or three across a road.
Head to moddb.com if you want to give it a try.