Coolant is the term used to refer to the antifreeze/water mix that flows through your car's cooling system. On most cars, checking the coolant is simple. There will be a bottle under the hood, with a cap on it labeled 'ENGINE COOLANT ONLY'. Just look at the side of this and see if the coolant level is between MAX and MIN. If it's too low, add antifreeze, if it's too high, don't worry, this won't usually cause a problem.