Client ZWAVE_GATEWAY-Mosquitto disconnected

I was checking my logs and found this. Can someone tell me what this means ?
The truth is that I don’t even know what Mosquito is and does. Do I even need Mosquito ?

2023-09-20 19:29:40: Client ZWAVE_GATEWAY-Mosquitto disconnected, not authorised.
2023-09-20 19:29:43: New connection from on port 1883.
error: received null username or password for unpwd check