Playing out from the back; the keeper being confident enough to pass it short to a defender whilst under pressure from the opposition players, rather than kicking it long to our small striker/wingers.
Caballero has been preferred to Hart as he's been deemed superior in this aspect, yet watching the Stoke game alone you'll see far, far too many instances where he kicks simple passes straight out of play. Could Hart do it? Unlikely given what we've seen of him over the many years he's been here, distribution has always been one of his weaknesses & clearly Pep has decided he won't be able to reach the required standard.
Watch any video of Bravo & you'll see he's far more comfortable doing this, watch any Barcelona game & you'll see the same, he's at home with that style of play. It's blindingly obvious why we're after a keeper like him.
Add on top of that the statistics you'll find on this thread that seem to indicate Bravo is a better shot-stopper than Hart & it's hard to come to the conclusion that this is a bad bit of business, especially for the quoted figures.