The phrase "coolant level" refers to the amount of liquid that keeps an engine cool. It checks if there's enough liquid so that the engine doesn't overheat. Full definition
In most cases, the radiator coolant level sensor is going to be accessible from the top of the engine compartment and with simply reaching down with your hands. (yourmechanic.com)
Check coolant level frequently and change it according to your owner's manual to prevent serious damage or engine failure. (carcare.org)
One of the symptoms of coolant leaks is low coolant level in the coolant bottle and coolant smell under the hood. (samarins.com)