A network is two or more devices joined so they can send each other data. The devices are called hosts (phones, laptops, servers, printers) and the connections between them are links: a cable, a Wi-Fi signal, a fibre-optic line under the sea.
Data doesn't cross a network in one long stream. It's cut into small pieces called packets, usually up to about 1,500 bytes each. Every packet carries a label with where it came from and where it's going, so it can find its own way and be put back in order at the other end. If one packet is lost, only that piece has to be sent again.
A network in one home, office or school is a LAN (local area network). A network that covers a city or a country is a WAN (wide area network). The internet is a network of networks: millions of LANs and WANs, run by different companies, all agreeing to pass each other's packets along.
Many conversations on a network are between a client, which asks for something, and a server, which answers. Your browser is a client; the computer that sends you this page is a server. The rules both sides follow, what to send and what it means, are called a protocol.
Two numbers describe how good a link is. Bandwidth is how much data it can carry per second, measured in bits per second (100 Mbps is 100 million bits a second). Latency is how long one packet takes to get there, measured in milliseconds. A connection can have lots of bandwidth and still feel slow if its latency is high, which is why online games care about latency.
Practice
Why is data cut into packets instead of sent as one long stream?