It is used to ignite the gas, that heats the water in your heating system. If the pilot light is not working properly, you should reset it.
Common problems with Ignition include:
If you have one of them, you’ll need to buy a new ignition by matching it to your device’s brand. Sparepartsmarkt has lots of ignition for various models of Bosch, Baxi, Viesmann, Ariston and etc. Just find a suitable ignition and your boiler will get to back its top form once again.