What is a freeze plug?

Answer 1

freeze out plugs are metal plugs in the block and heads of your engine. they are are there if the coolant was mixed wrong with to much water and the vehicle freezes they will pop out allowing expansion of freezing liquid hopefully saving the engine

Part 2: You will see these called freeze plugs, casting plugs, welsh plugs and probably other terms. The holes they plug are part of the cylinder block casting process to allow molten steel/iron/aluminum to rapidly flow into the block casting mold. The fact that they will pop out if the coolant in the engine freezes can be serendipitous but in most cases the plugs go bad due to galvanic action: the steel plugs installed at the factory are dissimilar to the engine block metal and will start to corrode (rust) and will start leaking after a while. This is why most antifreeze manufacturers claim they have anti-corrosion protection; it is also why 99% of replacement plugs are brass. Three points to remember: 1) before replacing plugs do your research and find out EXACTLY how many plugs your engine has and where they are. 2) if you have to replace one plug, replace them all. 3) if you are rebuilding an engine it is cheap insurance to replace the plugs.