Quality of Service. On the user side, it can be used to ensure high-priority traffic actually gets through first.

In networking, all of the data is bundled into packets. These packets are sort of like a shipping package; They contain a shipping label about where the data packet is going, and how soon it should be delivered. That latter part is QoS. If you have a compatible network, enabling and properly configuring QoS will allow the network to prioritize certain “urgent” data packets over other less urgent packets.

Maybe your large download is a low priority compared to your VoIP traffic. Because the download will still get done eventually if the packets get delayed by a few milliseconds, but if the VoIP packets end up waiting in line then you’ll get stuttering, bad call quality, dropped calls, etc… So QoS ensures those VoIP packets get delivered before the download packets do.

But on the ISP’s side, QoS basically means “we’re throttling the fuck out of you so we don’t have to actually build decent infrastructure.” Because if your neighborhood’s line can only handle 2000Mbps of total traffic, but the ISP has sold 3000Mbps worth of service, the ISP can use QoS to throttle everyone in your neighborhood and ensure that every user on that line still gets connectivity. It’s not the connectivity they were promised, but it’s enough for most people to not notice.

For instance, maybe you have three users with 1000Mbps connections. So when only two of them are using that 2000Mbps line, everything is fine. But when the third user connects, they find that they’re basically locked out. The line is already totally full, so all three users begin experiencing connectivity problems. To avoid this, the ISP uses QoS to throttle everyone; everyone gets throttled down to 666.66Mbps to account for that third user. No single user is getting the promised 1000Mbps, because the ISP has over-sold their infrastructure and is using QoS as a stop-gap to avoid actually upgrading. But since all three users can connect, and most won’t bother actually checking their speeds, the ISP is able to get away with it.