QoS, or quality of service, is a feature of network devices that allows network traffic to be prioritized according to the type of service, application, or device. This ensures that important applications such as VoIP (a technology that allows you to make voice calls using a broadband Internet connection instead of traditional or analog phone lines) or online gaming have enough bandwidth and low latency to run smoothly. QoS is key to optimizing network performance, especially in high network load environments.