The most fundamental truth is that we have information on one computer (Computer A) and we want it to be on another computer (Computer B). These computers are not in the same room.
How do we solve this? The simplest, most primitive way is to physically carry the information (e.g., on a USB stick). This works, but it's slow and doesn't scale. We need a direct, electronic connection.
So, we run a cable (like an ethernet cable or a fiber optic line) between Computer A and Computer B. We can now send electrical signals or pulses of light down this cable. We can agree on a simple code: a high-voltage signal means "1," and a low-voltage signal means "0."
We have just created the most basic form of a network: a direct link. We can now send bits (1s and 0s) between two computers.
This works for two computers. But what if we have three computers (A, B, and C) and they all need to talk to each other? We would need a cable from A to B, from B to C, and from A to C. For four computers, we'd need six cables. For 100 computers, we'd need 4,950 cables.
This is a failure of scale. Connecting every machine to every other machine directly is physically impossible.
The logical solution is to have a central device. Every computer connects to this one central point (let's call it a Switch). When Computer A wants to talk to Computer C, it sends the message to the switch, and the switch forwards it only to Computer C.
We have just invented a Local Network. A group of computers in close proximity (like an office or a home) can now communicate efficiently.
Now, our office has a local network. Another office across town also has its own local network. How does a computer on our network talk to a computer on their network?
We face the same scaling problem. We can't run a wire from every computer in our office to every computer in theirs.
The logical solution is to connect the networks themselves. We need a special, dedicated computer that is connected to our network and also connected to their network. Its only job is to pass messages, or "route" traffic, from one network to the other. Let's call this device a Router.
Now, if we want to connect to a third network, we just connect our router to their router. Suddenly, we can build a massive, sprawling web by connecting networks to other networks.
This is the fundamental truth of the Internet. The Internet is a "network of networks." It is not one giant cloud; it is millions of private and public local networks all connected by these special routing devices.