Distribution by AS shows traffic for specific end user by autonomous systems. Being aware of traffic users in your network generate towards other autonomous systems i.e. networks is of great importance in terms of preventing and resolving various situations concerning network security and reliability.
To view traffic distribution by AS:
In the image above, you can see that this user has notable amount of Facebook traffic in download direction, consuming large portion of available bandwidth between 12:03 p.m. and 12:15 p.m. as well as YouTube traffic in upload direction around 13:02 p.m.
To understand AS traffic in general, read more at AS View.