What is a host / end device, and what role does it play in a network?

"Host" and "end device" mean the same thing — any computer that connects to a network and participates directly in communication; it is where a message originates or is delivered.

Every computer on a network is called a host or end device. A host is either the source of a message or its destination: data originates with one end device, flows through the network, and arrives at another end device. Everything in between (cables, switches, routers) exists only to carry that traffic between hosts.

Examples: desktops, laptops, smartphones, tablets, printers, IP phones.

Go deeper:

• Host (network) (Wikipedia) — defines a host as a device that acts as source/destination of messages, and distinguishes hosts from infrastructure nodes.
• Key Players (Practical Networking) — frames hosts as the endpoints that originate and consume traffic, with everything else in between just carrying it.