Answer
You use schedulers to define the class-of-service (CoS) properties of forwarding classes and output queues. You configure CoS properties in a scheduler, then map the scheduler to a forwarding class. Forwarding classes are in turn mapped to output queues.
Schedulers determine the amount of interface bandwidth assigned to each queue on an interface, the size of the memory buffer allocated for storing packets in each queue, the scheduling priority of each queue, and the random early detection (RED) drop profiles associated with each queue to control packet drop during periods of congestion.
See https://www.juniper.net/techpubs/en_US/junos15.1/topics/concept/schedulers-overview-cos-config-guide.html