This is normally the coolant level warning light. If this comes on it means that the level of coolant in your radiator is low and needs topping up. DO NOT OPEN THE RADIATOR CAP WHEN THE ENGINE IS HOT! The coolant system is pressurised and it could easily release pressure and spray you with boiling coolant. Do it when the engine is cold. Top up the system with either a pre-mixed coolant bought from a shop, or with distilled water. Don't use tap water - the mineral deposits in it boil out in the cooling system and calcium gets depositted around the inside of the radiator making it less efficient (which will eventually cause it to fail). It's always best to use pre-mixed coolant, or to mix your own rather than using neat water. The coolant mixture behaves as an antifreeze in winter as well as a corrosion-inhibitor to stop your engine rusting from the inside out.
