When interacting with webpages, your browser is making dozens of requests to render a single page. The main request is for whatever URL is in your browser’s location bar, but there are many more that need to be made to get the images, videos, fonts and scripts that add functionality to a web page. In contrast to the actual information on a webpage, these things aren’t so dynamic.
The logo of a given site isn’t something that typically changes, and as such doesn’t need to have all of the heavyweight processing behind it than a newsfeed, or list of articles. Similarly, all of the frames of a video are known before someone even goes to a website, so it doesn’t require any processing at the time of someone going to a webpage.
Static File Servers are very fast and optimized HTTP servers that now how to send fixed pieces of data, like images (like logos), videos (like cats) and fonts to a browser without having any of the overhead of being able to handle responses dynamically. Just like how it’s always faster and cheaper to get a new suit off the rack instead of going to a tailor, static file servers allow developers to decide which parts of their webpage never need any tailoring, and serve them up very quickly.
The most popular static file servers are the Apache HTTP Server (colloquially, just apache) and Nginx. They can both be configured and customized to fit the particular needs of a project but out of the box can very quickly send information that doesn’t change to browsers everywhere.