Underdresses are worn beneath sheer garments. These can be different colors from the top dress, but they are generally not much heavier than the overdress but are a bit more detailed and more robust than a slip.
Those gathers in the above picture are at the back waist so my butt has a bit more room!
This one I made from a cotton sheet. I planed on wearing it with a sheer cotton number I got, but that dress needs some work, so right now this underdress is just folded in my lady things closet.