Your boiler has detected that the water level inside the heating system has dropped too low and needs to be topped up to function correctly.

DIY fix possible

Locate the filling loop (a flexible silver hose) connecting your cold mains pipe to your heating pipework.Open the valves on both ends of the filling loop to allow water to enter the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (usually between 1.0 and 1.5 bar), then close both valves tightly.