What is Pizzagate theory, which made Hillary Clinton flip out at her Epstein deposition?

During her Epstein files deposition, ex-US State Secretary Hillary Clinton flipped out on being asked about Pizzagate conspiracy theory, saying, "It was an outrageous allegation...can’t believe you're...referencing it." The theory falsely suggested that Democrats were running a child sex trafficking ring from a pizza joint in Washington DC. Some claims also suggest children were kept in basement.