The Maxim Tomato is an item from the Kirby series used to recover character's health. When eaten it fully restore a character's HP making it one of the best healing item in the games. They are apparently Kirby's favorite food.
Maxim Tomatoes are large red tomatoes with black "M"s on the front of them. The "M" likely standing for "Maxim".
Kirby Mass Attack
Super Smash Bros. series
Maxim Tomatoes also appear in the Super Smash Bros. series where, like the Kirby series, they act as recovery items. However, unlike the Kirby series, these don't fully heal a character; instead healing a set amount from a character's damage meter. In the original Super Smash Bros., they healed characters of 100% damage, while in later games after Super Smash Bros. Melee they only heal 50% damage.