system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 : I have bad service, how do ads still play perfectly?
gpt
So there are services called CDN. or content delivery networks. Their purpose is to let content providers store cached copies of their content on servers closer to where this content would be requested from. A lot of advertisers will contract the services of a CDN to put copies of their ads on servers that are usually installed directly in the data centers of service providers.

Because of this, the video you're trying to watch ends up being on a server on the other side of the country or in a different country, but that ad you're being forced to watch is probably coming from a server within your internet service providers internal network and doesn't even have to go out to the internet to stream the data to play it.