Sentence:  They can have a 15-card <WSD>sideboard</WSD> or no <WSD>sideboard</WSD>. The <WSD>sideboard</WSD> can be used to replace cards in the deck after each game. 
Meaning of the word marked with <WSD> tags:  A set of cards that are separate from a player's primary deck, used to customize a match strategy against an opponent by enabling a player to change the composition of the playing deck.