Mqtt and miflora to node red

Hello Community,

Can someone help me to explain how works the Mqtt Broker addon?! My hassio works with my mifloras and the Mqtt Connection works to the other raspberry but i don‘t know how i can set what for values are send the Mqtt. My project is that i will send the moisture value from the mifloras over the mqtt to another raspberry where run node red and there the relay should switch. But i don‘t understand the mqtt broker what i must set that it read this sensor values.
Please can someone help me :sob::sob:

Edit: Or have anyone a idea how i can switch the relay with the connected miflora sensor moisture values?