Low Coolant Warning Light
This light indicates that the engine's coolant system is running low on fluid and needs to be serviced. (This light may have a downward-pointing arrow.) It is pretty self-explanatory. If you see this light come on in your car, you need to add more coolant to your radiator. While putting water in your radiator is better than nothing at all, you should always place radiator coolant in your radiator when possible.
You should never drive your car while low on coolant. Doing so can result in a broken radiator or an overheated engine. If you drive with an overheating engine too long, you will ruin it and need to replace your entire engine, a very expensive procedure.
So if you see this light come on in your car, you should address it immediately.