Your boiler is always turned on - by which I mean it's ready for use. However, it's not active until your thermostats calls for heat. How and when it does that will depend on how it's been programmed.
Generally you can select Comfort (when you are in the house), Eco (when you are asleep or summer time).
You will need a setback timer and a room thermostat. Once the temperature in the rooms reaches the right temperature the boiler turns off again. When it falls below - and the setback timer is at an active period - it comes back on again.
Underfloor heating runs at a lower temperature than radiators. You use your thermostat to set the temperature you want. To understand why, imagine that your underfloor system is just like a very large radiator. The larger the radiator, the lower the temperature of the water within it (known as the flow temperature) needs to be in order to keep a room warm. Now imagine that that radiator is set in concrete, screed or some other solid material. In order for the heat to penetrate into the room, not only does the water need to warm the pipes it runs through, as with a regular radiator, it now also needs to warm that screed.
To get the pipes and the screed up to temperature takes a lot of energy. Repeatedly turning the system on and off, as you would with regular radiators, will waste energy.