Quality of Experience (QoE) describes metrics that measure the performance of a service from the perspective of a user or viewer. Common streaming video related QoE metrics include rebuffering, playback failures, and video startup time.
For example, when a viewer is streaming video, the network might experience a 2-second outage that would be captured by a QoS availability metric, however if the video player had buffered enough video data ahead of time the viewer won't experience any issues that would be measured by a QoE metric, like rebuffering.
Tracking Quality of Experience metrics can help you focus your optimization efforts on the parts of your system that will have the biggest impact for viewers.