Let's start with the basics. Antifreeze is an additive you add to a water-cooled engine to not only reduce the freezing point, but also increase the boiling point of the liquid inside the radiator and cooling system. This helps to keep the liquid in the host powerplant at a stable temperature, preventing it from freezing or overheating. Generally, when people talk about antifreeze and engine coolant, they're referring to the same thing, but regular water can also be referred to as coolant due to how efficient it is at preventing an engine from overheating. It's worth noting, though, using only water as engine coolant won't protect against sub-freezing temperatures. It also won't protect the engine's internals against the onset of corrosion, whereas antifreeze will do both jobs admirably.
How does it work, then? Most commercial antifreeze formulations include a glycol to suppress the freezing point and raise the boiling point. They also make use of corrosion-inhibiting compounds and a coloured dye (commonly orange, green, red, purple or fluorescent blue) to aid identification. A 1:1 dilution with water is usually
