A boiler can be controlled in two ways by a thermostat: on-off or modulating (also known as OpenTherm connection). Every boiler can be controlled on-off. Not every boiler can be modulated. Sometimes the boiler can be modulated, but a converter is still needed.
But what is the difference between on-off and modulating?
An on-off thermostat works like a light switch. The thermostat can only be "on" or "off". The thermostat switches on when the room is colder than the set temperature on your thermostat. At that moment your boiler starts. When the temperature is reached, your thermostat switches off the central heating boiler.
If you have a modulating thermostat you can have your boiler burn at different positions. This ensures accurate temperature control and that your boiler does not heat unnecessarily. A modulating boiler and thermostat have two-way communication. Your thermostat can therefore also receive information from your boiler.