Zigbee2mqtt is open source software that runs on a PC or laptop. Many run it on a raspberry pi.
The device that runs the Zigbee2mqtt software also needs to have a zigbee adapter installed.
The getting started guide on the official site covers this along with recommended hardware.
Zigbee2mqtt acts as a gateway between a zigbee network and an MQTT network.
You can manage/view your Zigbee devices from home assistant via MQTT.
For this to work you will need to have installed and configured the MQTT integration
and enabled automatic discovery.
When the devices joins the Zigbee network the device details will be sent via MQTT and discovered by Home assistant.
The Zigbee2MQTT gateway creates a config message using the device details and sends it to the broker as described in Adding an MQTT Device to Home Assistant
Below is an example config message created by the Zigbee2MQTT gateway:
Notice the topic of the message. Home assistant will know from the payload what topics it needs to subscribe to to receive data from the device.
The Zigbee devices will appear as devices in Devices
and can be added to the dashboard.
Related Tutorials and resources.