According to this article, "Level 3 is also known as an 'eyes off' system, where a person sits behind the wheel, but can take their eyes off the road ahead while the car does all of the actual driving. Level 3 autonomy allows the car to make situational decisions, but a driver must still be alert and ready to take over where necessary."
In other words, it's is a high level of advanced driver-assistance where less input will be needed from a human driver. However, fully autonomous driving will only be achieved once vehicles can effectively communicate with one another and there is no need for driver input.