It all starts with a pretty basic concept that you probably take for granted- whenever you access a site online, a physical computer somewhere in the world has to send a signal to your physical computer. It can be helpful to imagine that the whole internet is a very fast version of regular postage- if your friend has a website you want, you write a letter to them asking for it and then they send a letter back to you with it inside.
But before you write a letter, you have to know where to send it. How does your computer know what computer to talk to? Well, at first, it doesn't. All it knows is it's looking for something called "facebook.com." So it reaches out to a computer that your router has heard of called a Domain Name Server. This is what DNS stands for. Domain Name Servers (there are lots of them) look at domains (such as facebook.com, cracked.com, anything you'd type in your URL bar to go to a website) and turn them into addresses. You've probably heard of an IP address by now- Domain Name Servers basically keep a list of domains and their associated IP addresses. If the DNS you reach out to doesn't know where to find a site, then it will reach out to a computer that does.
Earlier I mentioned that Facebook has an internal network they call their backbone network. That network is private, meaning Facebook doesn't want all of their computers' IP addresses on other peoples' Domain Name Servers. So Facebook has their OWN DNS that everyone reaches out to, and their private DNS figures out where to send their request. This is pretty common for medium and bigger tech companies.