WaPo filed a document with the court asking it to order Mueller to turn over sealed docs from the Manafort case. This filing triggered a deadline by which Mueller must respond.
However, Mueller says he needs more time to get a response together because he's too busy with other matters right now. Accordingly, Mueller needed to ask the court to extend his deadline to file a response.
Mueller reached out to WaPo to determine whether it would challenge his request for an extension, and WaPo decided not to challenge Mueller's request for extension (unopposed).