We are going to begin with an analogy, and a review of traditional mail systems. If you, Jane Doe, want to mail your brother John Doe a message. What do you do?
It starts with a name. When John and Jane were born, they were given a number. A Social Security Number. This number is registered with 'The US Mail' system.
But that number is a SPECIAL number. It IS John. ONLY John can have that number. And we can't have just anyone be 123-45-678. Besides, who wants to remember that anyway? So we give person 123-45-678 a name: John, of family Doe.
Person 123-45-678, John Doe, exists and is known by the world. He is 'Registered'. But where? Where does John physically reside? In a house, on a street, in a city, in a state, with a zip code. John can now 'Register' his name and physical location with 'The Mail' and he can now send and receive mail!
The internet works EXACTLY like this. The Social Security number is a device's MAC address. The name? IP address. City, state, country? Router, Switch, ONT/modem. That's the magic of Network Engineering in a nutshell.