an end system
Picture a phone network: the phones are the things people actually talk into; the wires and exchanges in the middle just carry the call. In a computer network the equivalent of the phones are the end systems — the devices at the edge that run the applications people use. Your laptop, your phone, a web server, a smart TV, a sensor in a factory: each is an end system, also called a host because it hosts (runs) applications.
End systems are where data is created and consumed. They are deliberately separated from the network core (the routers and switches that just relay data). A host typically does the hard work of breaking a message into packets, attaching addresses, recovering from loss, and reassembling — while the core just forwards. Hosts come in two famous roles: a client asks for something (your browser requesting a page), and a server provides it (the machine sending the page back). A single machine can be both at different moments.
The line matters because of a design principle: keep the network core simple and dumb, push complexity out to the smart end systems (this is the end-to-end principle). It is why the Internet could grow so fast — new applications (the Web, video calls, an app you wrote last night) only need new software on the end systems, not changes to the millions of routers in between.
Your laptop (a client end system) and a server in a data center (a server end system) exchange data; the dozen routers between them are not end systems — they never open your message, they just forward it.
End systems run the apps and live at the edge; switches in the core just relay.
Routers and switches are computers too, but they are not end systems — they relay other people's data rather than being the source or sink of an application's messages.