Swallow, otherwise known as Inhale, is a move found in the Kirby video game series. It's one of Kirby's hallmark moves, and has been a paramount addition of the series ever since the beginning. The move was updated in Kirby's Adventure for the NES, which was allowing Kirby to not only suck enemies in, but to also copy certain enemies' abilities. This move was later brought into the Super Smash Bros. series where he'd be able to both inhale enemies and steal their abilities (example: If Kirby sucked in Yoshi, he'd gain his Egg Lay ability).
In the Kirby series, this is Kirby's initial and main attack. In the first Kirby game, Kirby's Dream Land, Kirby couldn't gain powers by inhaling them, only swallow them or spit them out in a star-shaped attack to destroy enemies and blocks. Kirby's Adventure started the series staple of powers from enemies. Kirby Super Star introduced a function where a sucked up power cold be converted into a partner, either controlled by AI or another player.
Super Smash Bros.
The Swallow move sucks in opponents who get in range, much like in the game. Once, sucked up, Kirby has the choice of completely Swallowing them to gain the opponent's neutral special or spit them out as a star like in the games though with a significantly smaller range. Opponents just pop out after Kirby chooses. Kirby also has the choice of keeping the opponent in his mouth but he is much slower and heavier and the opponent can get out. This way has the option of using the suicide tactic where Kirby walks off the edge with the opponent in his mouth. If the opponent doesn't escape fast enough, he loses a stock whereas Kirby does have a chance at recovery thanks to his excellent jumping ability.
In addition to copying their standard special, Kirby gains a unique hat based on the character he sucked up. Since Kirby gains one for each character in the game, there are over 50 Kirby Hats. Melee had a couple trophies dedicated to it.
In Super Smash Bros. For Wii U / Nintendo 3DS, this move has 2 additional variations from the normal use for customized Movesets
The second customization is a launching Swallow. Kirby starts the animation the same but, instead of sucking in the opponent that walks close enough, Kirby launches himself to swallow the opponent. This is likely good for the suicide tactic