MQTT reading

Setting up MQTT is quite technical, so it all assumes you know what you want to do.
I suppose you have to frame your search in openHAB terms.
I’d like to get some data into an Item.
To get that from outside, you will need to use a binding - you’ve already found out to use the MQTT binding.
To get the binding to send the data to your Item, you need to set up Things and channels.

This looks relevant -