In the world of "Fallout," a small subsection of humanity survived a nuclear holocaust by taking shelter in massive, radiation-sheilded vaults — essentially, entire small-scale underground cities made to sustain mankind until it was safe to go outside.
Vault 76 has been mentioned in every "Fallout" game to date, and is referred to as one of the first few bunkers to open after the nuclear war.
On Reclamation Day, Vault 76 opens.