Your boiler has detected a flame when it shouldn't be there, which usually means a internal component is stuck open or there is a fault with the electronics.

Flame On Before Gas Valve On

What causes this fault?

This fault occurs when the boiler’s safety sensors detect a flame inside the burner before the gas valve has actually been told to open. The most common reason is a gas valve that is physically stuck open or leaking, though it can also be caused by a fault in the control board’s electronics that incorrectly senses electricity where there shouldn't be any.