In geographic information system (GIS), overlay is an operation that superimposes layers of geographical data —like maps, satellite photos, roads, borders, etc— on top of each other.
Some satellite photos of Earth at night show country borders and shorelines. Flat-Earthers use the fact to dismiss the images as fake. They claim there are no lights illuminating the border on the ground, therefore the borders should not be visible. In reality, the borders are just an overlay added on the top of the original image. They are not part of the original satellite image.