Quality of Service (QoS) is a service that allows you to prioritize your internet to your VoIP devices. Most users won’t need to meddle with it,

For example, other employees may browse the internet, stream videos, and listen to music, or some software will do extensive file sync. All these services take up bandwidth, which all compete for your internet’s phone service.

QoS ensures that enough bandwidth is available for your VoIP phone system, if you do face bandwidth issue, you will have to setup QoS on your router.