Pellets are the food that snakes must eat to grow bigger and longer. The bigger the pellet, the more it will contribute to a snake's growth.
Pellets spawn at random on the ground, but they are also generated by players who use their movement burst. Pellets will also be generated when a snake dies. The pellets will spawn in clusters of around the same size the snake's body segments had when it died.
There are also special pellets that appear larger than most other pellets and move around the screen, generally trying to avoid the player. When eaten, these special pellets give a much larger amount of length to the player and so it can be worth it to use your speed ability to grab them, especially early on in a game.