Traction is the sustained attention of players, particularly fans who evangelise your game.
Whether you are a mega-publisher, a portfolio publisher or a single franchise publisher, the digital economy increases available choices to customers by a factor of 1000. So while the cost of distribution drops to nothing, the competition for attention skyrockets.
The first job of any game on any platform is therefore to achieve significant traction, irrespective of average revenue per user (ARPU). As a baseline, this means gaining the attention of 100,000 players for 60 days.