Nuggets are resources found in Pikmin 3 and come in piles. They are also only found in Mission Mode. There is a number displayed above the pile, signifying how many are left. Each one is worth 3 Pokos, so multiplying the number by 3, that will be how many Pokos you get. As long as Pikmin are left alone while collecting nuggets, they will continue to return to the pile until all the nuggets have been removed.
If Pikmin are called back while in the process of carrying a nugget, the Pikmin will go back to following the leader and the nugget will disappear and go back to the pile.