A Hidden Ability is a third, rarer Ability slot for a species, introduced in Generation V through the Dream World, that a Pokémon normally cannot be obtained with through standard wild encounters. Many Hidden Abilities, such as Speed Boost Blaziken, are stronger than the regular options.
Generation V introduced Hidden Abilities in Pokémon Black and White, first distributed mainly through the Dream World, an online feature that closed in 2014. The idea survived the feature. Later games moved Hidden Abilities into SOS battles in Generation VII, Max Raid Battles in Generation VIII, and Ability Patches and special encounters in Generation IX. The Ability Patch, added in the Generation VIII downloadable content, can change a Pokémon's Ability to its Hidden Ability directly.
The slot matters because the third option is often the best one. Blaziken with Speed Boost gains Speed every turn and plays nothing like a Blaze Blaziken. Greninja with Protean changed its type to match each move it used before Generation IX reworked the Ability to trigger once per switch in. Espeon with Magic Bounce reflects status moves and entry hazards back at the user.
Breeding passes Abilities down, and a female with a Hidden Ability could pass it to her eggs, which made one legitimately obtained Hidden Ability Pokémon the start of a whole breeding line. Males and gender unknown Pokémon can also pass it when bred with Ditto in later generations. Check the slot before you judge a species, because the hidden option often defines it.
In play"I need the Hidden Ability on this Torchic or the whole set does not work."