"Hide the getaway car in a discreet location" is an objective in the story missions in Grand Theft Auto V.
Next you need to park it in a discreet location. These are the places you would like to hide from the cops: dark alleys, odd places. It cannot be within a few blocks of any law enforcement agency and it has to be out of the way. All you need to do is stop for a few seconds and the game will tell you.
When a good place is found, call up your phone and scroll to one of the other protagonists. Above "hang out" will be a new option to "mark getaway location". Choose this option and you are good.