A network model divides communication into discrete layers. Each layer handles one task and communicates only with adjacent layers.
Think of mailing a letter. You write the message (application layer), put it in an envelope (transport layer), add an address (network layer), and hand it to the post office (link layer). Each step adds information.
Without layers, changing one component means rewriting everything. Layers let you swap Ethernet for WiFi without touching TCP. The two main models are OSI ( layers) and TCP/IP ( layers).