Boost
From Castle Wiki
Boosts are a way to promote a deck to reach more people.
Once a deck is boosted, it will temporarily get a higher number of impressions in the Castle feed, and it will also appear in the Boosted Decks row on the Explore tab. The person who boosted the deck will have their username featured in the Boosts tab for that deck.
Everyone can keep boost items in their Inventory to use on decks. To get boosts in your inventory, go to the Market and exchange Bricks for boosts.