What is a maternity pad?

Hello mummies, Mind sharing with me what's the purpose of the maternity pad? When should I start using it? Is it for post natal or when the due date is drawing near? What's the different between the normal sanitary pad and the maternity ones? Not possible to use panty liner instead? My EDD is on 14th Oct and this is my first pregnancy. Please help a sister out. Thank you so much 😇💕