Breeding is the system, introduced in Generation II, in which two compatible Pokémon left at the Day Care or Nursery produce an egg that hatches into the lowest stage of the mother's evolutionary line. Compatibility is set by Egg Groups, and parents pass down IVs, Natures, Abilities, and moves.
Pokémon Gold and Silver added the Day Care breeding system, and it quietly became the engine of serious team building. Leave a male and a female that share an Egg Group, walk, and an egg appears. The baby is the mother's species at its earliest stage, inherits moves the parents know that it can learn as egg moves, and inherits some IVs from both parents. Ditto breeds with almost anything, which makes it the most useful breeding partner in the games.
Later generations added control. From Generation IV an Everstone passes a parent's Nature. The Destiny Knot, from Generation VI, passes five IVs from the parents instead of three. Power items passed a specific IV before that. A parent with a Hidden Ability can pass it, with the rules loosening across generations to include males bred with Ditto. Flame Body and Magma Armor in the party halve hatching steps from Generation III onward in most games.
Generation IX replaced the Day Care with picnics in Pokémon Scarlet and Violet, where eggs appear in a basket while compatible Pokémon picnic together. The goal never changed: hatch dozens of eggs, keep the one with the right Nature, Ability, and IV spread, and release or trade the rest.
In play"Breed that Ditto with everything until you get a five IV baby to start the line."