OpenTherm is a protocol that allows a boiler to modulate. That is, the thermostat tells the boiler how much heat is required based on the current demand, and then the boiler lowers or increases the temperature of the water it pushes around the system automatically. This helps save another few percent on energy bills. However, to get OpenTherm to work, you need to have a boiler that supports it and a smart thermostat that supports it, too. Most modern thermostats do, but if you've got an older boiler, you may find that it's not so well supported.