Your boiler has detected that the water pressure is getting too low to run effectively, which will eventually cause the heating to stop working.

DIY fix possible

Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.Slowly open the valve or valves on the filling loop until you hear water entering the system.Watch the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the valves completely.