It's used for ? Coolant is used to cool engine parts just to prevent overheating.
If the warning light comes on: stop right now! Your engine may overheat, damage other parts, and fail.
To prevent it from turning on:
Change the coolant regularly; according to the indications in your car's maintenance book
Bleed the cooling circuit
Check that there is no leak in the tank
Check the coolant level
If you need to fill the tank, keep the engine cold