The Priceless Maracas is on of the 76 side-quests in The Legend of Zelda: Breath of the Wild. It can be activated by speaking with Hestu on the road in the Dueling Peaks Region just south of Kakariko Village. Upon completing the side-quest, Link will unlock the ability to upgrade his inventory stashs, by trading in Korok seeds that he finds all over Hyrule with Hestu.
Following the winding path from Kakariko Village and you come across a large korok named Hestu. He will inform you that he lost his maracas and tasks you to find them. The find the maracas, walk back to a henge-like rock and beyond that will be a Bokoblin's camp. Defeat the Bokoblins and unlock the treasure chest to obtain the maracas. Return them to Hestu, who in turn will reveal that there are no seeds inside to allow him to play his instruments. At this point, side-quest will be completed and allow Link to begin trading in the korok seeds that he or has already found to upgrade his inventory.