From the Super Mario Wiki
Pellets are Pikmin items found in Super Smash Bros. Brawl, Super Smash Bros. for Nintendo 3DS, and Super Smash Bros. Ultimate on the stage Distant Planet. They can be knocked off of Pellet Posies, picked up, and thrown at opponents to cause some damage. There are red, blue and yellow Pellets, with various numbers on them, although these factors make no difference to any damage they may deal. When thrown into an appearing Onion, they release items.
Super Smash Bros. for Nintendo 3DS / Wii U Trophy Description