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.
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.
Boosting a deck will generate a special comment on the deck, and will cause your avatar to appear among the fans for that deck in the Hype tab for that deck.