The Heart Container (ハートのうつわ, hāto no utsuwa), known simply as Heart in Smash 64, is an item that will reduce the damage of the character who picks it up. In Super Smash Bros., the Heart Container will reduce a character's damage meter to 0%, but in all Smash Bros. games after it, it only heals 100% worth of damage. In the original game and Melee, a player cannot take any damage for the short amount of time that the heart spends replenishing health, though in Brawl, any damage received during healing will be subtracted from the 100%. In Melee and Brawl, the frequency of a Heart Container appearing on stage is quite rare compared to other items.
In the All Star and Boss Battles modes, there are special Heart Containers which will heal 999 points of damage instead of 100 points. In Melee and Brawl, those modes have 3 special Heart Containers (5 if playing Co-Op), whereas in Smash 4 they have only two, along with a Fairy Bottle and a Maxim Tomato, making for more strategic usage.
The Heart Container made its first appearance in The Legend of Zelda. At the beginning of The Legend of Zelda, Link starts off with a maximum health capacity of three hearts. When Link finds a Heart Container, it gives him another heart of capacity, allowing him to sustain more damage before dying. Starting in The Legend of Zelda: Link's Awakening Heart Containers would also replenish Link's health to full upon collection. In Super Smash Bros. and Melee, the Heart Container's design is based around Ocarina of Time's Heart Containers. Its appearance changed in Brawl to the design in The Legend of Zelda: Twilight Princess, like most Zelda content.  The design of the Heart Container changed once again in Super Smash Bros. 4 to match its appearance in The Legend of Zelda: Skyward Sword, giving it a thicker and more detailed aesthetic.
In the Super Smash Bros. series
In Super Smash Bros., a Heart Container will heal all damage up to 999%.
In Super Smash Bros. Melee, Heart Containers reduces 100% damage when used in battles.
In SSBM's All-Star Mode, every character must be fought consecutively with only three special Heart Containers used to heal between matches. In this mode, they will completely heal the character. If the player enters the teleporter before they finish healing, it stops prematurely. If the player makes it through all of All-Star mode without using a single Heart Container or a continue, then they receive the Melee Master bonus, which is worth 100,000 points.
Heart Containers in Super Smash Bros. Brawl are the same as their Melee counterparts, except the item's design now uses the design from The Legend of Zelda: Twilight Princess. SSBB's All-Star and Boss Battles Mode both include 3 Heart Containers, but 5 are available if co-op play is chosen.
In All-Star and Boss Battles Mode, Heart Containers heal all health, the same way they do in Melee, and do not stop healing too early if the player enters the teleporter before it's finished.
The Heart Container returns once again in Super Smash Bros. 4, using its design from The Legend of Zelda: Skyward Sword. It functions the same as in Melee and Brawl, including its special version found in All-Star mode.
Description from the Super Smash Bros. Instruction Booklet
The Heart Container recovers all of your strength, returning your Damage Meter to 0%.
Heart Containers drift toward the ground slowly and replenish 100 percentage points of health. In The Legend of Zelda, these items beefed up Link's life meter; in most cases, Link found them in the possession of bosses or hidden in secret areas. Heart Containers are often scattered across the land in several fragments.
The best recovery item! Grab this and heal 100 percentage points of damage. It falls slowly, so it's best to grab it in midair. Damage recovery won't happen instantly, though, so try not to get launched while your damage is counting down. In the Legend of Zelda series, Heart Containers raise the player's maximum health. Bosses always have one in their possession.