QoS = Quality of Service.
QoS is something that sets packet priorities. Obviously the ones for adverts are higher and all your non-important stuff gets put to the back. QoS also reserves bandwidth just in case something with high priority might come along, so you get ads faster than downloads and ads are the first thing to show up on the page when it starts rendering.
Not that most routers pass QoS info through, some don't even forward that data, so all you do is introduce latency by spending time flagging packets before they go to ethernet. The whole shit is so fucking useless and broken and designed to work against the consumer, it pisses me off. Luckily MS was nice enough to enable that shit by default on all Windows installs.
On an internal network QoS may be put to some good uses, but unless you're a business, on the internet QoS is just used to shun your data. VoIP and shit like that work fine without it, and the people that warn you to use it and shit are on crack.