Radiator coolant actually raises the boiling point of the water, allowing it to carry more heat away from the engine, and that means cooler running. If you're running low, things can get steamy fast. Think of your coolant as a spring dance chaperone, there to keep things nice and cool even when you're doing a lot of moving around.
Luckily it only takes a second to check your coolant level. Most cars these days have an opaque coolant overflow tank right next to the radiator. It's white plastic, but you can see inside to see if there's enough juice to keep things safe. You'll also see markings on the side telling you the low and high levels to work with. Always check your coolant level when the car is cold.
If your levels are good, no need to go any further, enjoy the peace of mind. But if you're low, you'll need to top it off. Your engine takes a 50/50 mixture of coolant and water. In the old days, you had to make measurements yourself and test the mixture to get it right. But since we live in the age of convenience, you can now buy premixed coolant that's ready to pour. For a simple topping off, I recommend going this route. It might cost a buck more, but you're in for much less mess.
To add the coolant, simply unscrew or pop off the cap on top of the opaque plastic overflow reservoir and add your mixture until it reaches the full mark. Now put the cap back on nice and tight and you're ready for any weather.
*Safety Point: If you spill any coolant on the ground while you're filling, be sure to wipe it up. Coolant is very toxic to animals, but they like to drink it because it tastes sweet. Save a little furry life!