Google has slowly started rolling out a feature that lets you swipe to unlock your phone while using Android Auto. Previously while connected to Android Auto you would not be able to access anything from the screen of your phone, the only way to do so would mean unplugging the USB and disconnecting (for the upcoming wireless option) from Android Auto.
The main purpose of Android Auto is to reduce distracted driving by allowing users access to key functions of their phone without taking their eyes off the road while driving, so it is understandable why they initially implemented this lockout. While this does limit the ability to be distracted by your phone while driving it was also one of the top complaints about Android Auto. Google has yet to make an official announcement about this new feature but users have been reporting access to this new feature already.