Quality of Service, or more commonly known as QoS, is a technology that relies on assigning different levels of priority to different types of IP traffic flows.
Higher prioritised IP traffic flows are given preference on the network, reducing packet loss on congested links and help control latency plus jitter.
Lower prioritised IP traffic are seen as less essential in times of congestion on the network, their delivery across the link is seen as best effort only. Best effort can result in traffic not arriving in sequential order, delayed due to congestion across a link, or dropped altogether.
Quality of service is implemented on a network to try ensure efficient delivery of traffic for applications that need real-time delivery. The traffic could be for IP telephony, video conferencing, where a delay or loss in traffic is noticeable quickly.
Quality of service can also ensure critical services can function over congested links, like routing protocols.
Non-business critical traffic can be dropped over business critical traffic.
Trust boundaries can be set over the network to accept or reject traffic based on markings injected on the packets.