Your boiler has detected that there is too much water in the central heating system, causing the pressure to rise above a safe level.

DIY fix possible

Check that the filling loop valves are fully closed and not letting in more waterBleed water from a radiator using a bleed key to reduce the system pressureMonitor the pressure gauge to ensure it returns to the normal range (typically 1.0 to 1.5 bar)Reset the boiler once the pressure has been lowered