Zigbee coordinator, router & end device
Every Zigbee network has exactly one coordinator — the radio in your hub or USB dongle that forms the network and stores its keys. Mains-powered devices (plugs, bulbs) join as routers that relay traffic and extend range; battery devices join as end devices that sleep between transmissions and route nothing.
The three roles explain most Zigbee network behavior. Range problems are router problems: a lone battery sensor at the far end of the house needs a plug or bulb partway along to relay through, because end devices cannot forward traffic. A healthy mesh has routers spread through the home, and adding a smart plug is the standard fix for a flaky sensor.
Because the coordinator holds the network, it is a single point of failure and identity: replace it and, in general, you re-pair everything — though coordinator backup and restore between compatible radios is routine on Zigbee2MQTT and ZHA. Dedicated coordinator dongles such as the Sonoff ZBDongle-E or SMLIGHT SLZB-06 are how DIY platforms like Home Assistant get their Zigbee radio.