Sun is the resource spent to place plants in the original Plants vs. Zombies.
During play, sun appears as collectible icons that fall onto the lawn during daytime levels. Players can click those icons to add to their supply, then spend the accumulated resource on plants whose costs vary by strength and function.
Sun-producing plants are central to the game's economy. Sunflowers generate additional sun, while Sun-shrooms provide a lower-cost nighttime alternative that grows more productive over time. Managing production against immediate defensive needs is one of the original game's core strategic decisions.
The answer is sometimes confused with coins, but coins are mainly an in-game money system used for Crazy Dave's shop and other upgrades. Brains are the objective in the I, Zombie puzzle mode, and gems belong to later franchise systems rather than the original placement resource.