What feature did Pokémon Gold and Silver introduce for raising Pokémon eggs?

Pokémon Gold and Silver introduced Pokémon breeding to the main series.

The 1999 Game Boy Color games added the Pokémon Day Care, where compatible Pokémon could produce an Egg. Players could then walk with the Egg in their party until it hatched, adding a new way to obtain Pokémon and pass on selected traits.

Breeding became important for collecting Pokémon, obtaining baby Pokémon, and developing competitive teams. The games also introduced Eggs as story and gameplay objects, including the Egg received from Mr. Pokémon near the beginning of the adventure.

Breeding is distinct from evolution: an Egg produces a new Pokémon, while evolution changes an existing Pokémon into another species. Pokémon Gold and Silver also introduced Pokémon holding items, day-and-night cycles, and the Johto region.