Your boiler is checking if there is enough water pressure to run safely and has paused briefly to confirm the system is full.

DIY fix possible

Check the pressure gauge on your central heating system (it should be between 1.0 and 1.5 bar).If the pressure is low, use the external filling loop valves to top up the water level.Ensure all radiator bleed valves are closed and any air has been vented from the system.