An Orthomosaic Map is essentially a map created by taking many overlapping images over an area. The drone is programmed to fly a specific pattern over a given area and will take many georeferenced images usually with the camera pointed straight down.
The images are then processed and stitched together to produce one large "map" of the area. These maps can be used to identify change in environment, progress of a construction project, or as an up-to-date map of a piece of property.
Since the images are taken from a drone that is only flying 200 feet above the ground, there is much greater detail than satellite imagery.