The bedstead is the wooden or metal frame of a bed — the part that holds the mattress up. It is not the mattress, not the sheets, and not ‘the bed’ in the loose everyday sense (when we say bed we often mean the whole thing you sleep in). You meet the word in furniture talk and in older writing: an iron bedstead, a brass bedstead, a wooden bedstead. In ordinary speech many people now just say bed frame; bedstead sounds a little dated, or like a shop catalogue, though it is still the clear word when you need to name the frame alone.