QAnon is a kind of mega-conspiracy theory popular with fringe-minded Trump supporters like Barr. It combines recent conspiracy theories like Pizzagate and the Seth Rich murder conspiracy, as well as classic conspiracy elements like fear of the Rothschild family.
QAnon believers are convinced that the world is run by a nefarious deep state cabal of Democrats, celebrities, and intelligence community figures (many of whom, they claim, are pedophiles).
Trump is about to take them all down, in their telling, often with sealed indictments that are hidden from the public. Hence Barr’s tweets about massive pedophile networks.