It's dillinger's way of describing the gossip model. The idea is that you look in an npub's prefered publishing "outbox" for their posts (type 1) and you could "dead drop" to a relay their profile indicates where they prefer to rendevous to pick up their incoming DMs (i.e. "inbox") 

 see, these people hate on mike even though he came up with this as a way to improve user experience

i may hate his choice of programming language but he did it because you all dumbasses never properly supported NIP-65 either!

SLAP! 

 I like mike