Whereas queuing provides congestion management, mechanisms such as WRED provide congestion avoidance.WRED’s purpose is to prevent an output queue from ever filling to capacity,which would result in packet loss for incoming packets.
To understand what it is WRED ,let’s explain first RED goals, basics
The purposes of RED is to prevent TCP synchronization (all TCP streams in slow start phase) by randomly discarding packets as an interface’s output queue begins to fill.
The agressivity of discarding packets depends on the queue depth and is influenced by the minimum , the maximum threshold and the Mark Probability Denominator(MPD).
So random dropping will happen when you reaches the minimum threshold and it is full when you reach the maximum threshold .
Another definition for the MPD is the maximum drop percentage at the maximum threshold.
RED is not supported on Cisco IOS but Cisco has chosen to support CB-WRED which gives you the opportunity to define several RED mechanisms for different DSCP threshold.