The filling loop/tap is a small braided hose with a valve attached. Its aim is to allow you to add additional water to the boiler system. As the volume of your system is fixed, adding more water increases the pressure on boiler.
Do I need to turn my boiler off to reppressurise?
The most effective way to handle a problem with low boiler pressure is to repressurise the system.
You can usually do it by yourself without calling any service but do not forget that you should first check the user guide to make sure that your model allows this.
Can I repressuries my boiler when it’s working?
While your boiler is filling with water, look at the pressure section. When the needle is just under 1.5 bar, turn the handles to closed again.