when an engine block is designed; there is special kind of molded sand the shape of the coolant passages in the engine block. This sand sculpture is created inside a mold and molten iron or aluminum is poured to form the engine block. When the casting is cooled, the sand is removed through holes in the engine block casting leaving the passages that the coolant flows through. thus, if the holes are not plugged up, the coolant will pour right out.
Plugging these holes is the duty of the freeze-out plug. These plugs are steel discs or cups that are press fit in the holes in the side of the engine block and usually last the life of the engine without problems. However there is a reason they are called freeze-out plugs. In the early times, many people used plain water in their engines, usually after replacing a burst hose or other completing cooling system repair.
However people are forgetful and many a motor suffer the fate of the water freezing inside the block. Often, when this happens the pressure of the water freezing and expanding forces the freeze-out plugs to pop out, relieving the pressure and saving the engine block from cracking. (Although most times, the engine cracks anyway). Another reason for these plugs to fail is the fact that they were made of steel and would easily rust through if the vehicle owner was careless about the cooling system maintenance. Antifreeze is designed with rust inhibitors in the formula to prevent this from happening, but those chemicals would wear out after 3 years, which is why antifreeze needs a periodical change. Because some people leave plain water in their engines greatly accelerated the rusting of these freeze plugs.
When a freeze plug gets so rusty that it perforates, you have a coolant leak that must be repaired by replacing the rusted out freeze plug with a new ones. This kind of repairs ranges from fairly easy to extremely difficult depending on the position of the affected freeze plug. Freeze plugs are found on the sides of the engine, usually 3 or 4 per side. There are also freeze plugs on the back of the engine on some other models and also on the heads.
As long as you are consistent about maintaining the cooling system, you need never worry about these plugs failing on modern cars.