What a normal map does
A sprite is flat, so a light in a 2D game brightens it evenly, like a sticker. A normal map is a second image of the same size whose colours say which way each pixel faces. With it, the engine lights the side of the sprite that faces the lamp and shades the other side.
No drawing holds that information, so it has to be guessed. This tool guesses a height for every pixel and turns the slopes of that height into the map.