Stands for "Quality of Service."
If there are a lot of devices on a network, all fighting for a limited amount of bandwidth, speed and latency can degrade for them all. Configuring QoS settings on the network's router can prioritize certain devices and types of traffic, ensuring that the most important data reaches its destination quickly.
QoS features vary, depending on the capabilities of the router. Some routers can prioritize network traffic by type. For example, video calls and streaming media using the UDP protocol rely on their data packets arriving as fast as possible for a smooth experience, while other traffic like web browsing and file downloading use the TCP protocol that is less reliant on speed and low latency. Enabling QoS can prioritize time-sensitive services, giving them all of the bandwidth they need at the lowest latency possible. Less powerful routers lack the ability to prioritize by traffic type, but still can use QoS to prioritize certain devices on a network above others, by either IP or MAC address.
When QoS is enabled, a router manages traffic by focusing on several factors. First, it allocates bandwidth to give certain traffic a larger percentage of available bandwidth over lower-priority traffic. It also manages how the router queues incoming and outgoing data packets, making lower-priority packets wait while the higher-priority ones get to move first. If there's significant congestion on the network and some data packets need to be dropped from the queue, QoS rules also drop the lowest-priority packets first.