Pellets (aka agar, rainbow pellets) are little dots in different colors that spawn randomly all over the map. Initially, they give 1 mass when eaten, but can grow in size over time and give more mass then.
Pellets can only be eaten by players, if a virus is shot over a pellet, the pellet will remain under the virus. If a spawner virus ejects a pellet into a wall, the pellet will slightly rebound off the wall and fall into the spawner, but not eaten by it. If ejected mass is shot on a pellet it will stay in the same spot, only covered by the ejected mass.
Eating pellets is the most common way to gain mass. The other ways include: eating other cells, eating a virus (when large enough to eat them) and eating W mass ejected from other cells. Since cells lose mass over time, big cells can not eat pellets to gain more mass. (They can only eat pellet to slow down losing mass.)