You need a copy of Clear The Decks to use these mats.
Clear The Decks! Playmat adds a beautiful background for you card stacks in game, printed on a non-slip mat to organise your cannon cards. Each mat is for single player use, mats are optional and not having one does not prevent gameplay.
If you play Clear The Decks solo, you will only need the one, otherwise you will need one for each player.