QoS (Quality of Service) is a system designed to guarantee a minimum bandwidth to users through traffic prioritization techniques. If we relate it to the context of video streaming, we refer to the capacity of a network to effectively and efficiently deliver video bytes.
It is often used to control traffic on a network. It is not the same as a bandwidth limiter, since the latter does not perform any kind of prioritization on the packets in queue.
A QoS usually solves problems related to low throughput, high latency or communication failures. Many routers on the market choose to incorporate this system, as it is quite effective and gives good results.
This is because the content to be enjoyed on the network is very broad, and each type has its own particular needs to ensure maximum user satisfaction. Playing a streaming video, which requires high bandwidth, is not the same as playing a video game online, where we want the lowest possible latency.