A freeze plug is a metal or rubber plug that is inserted into a hole in the engine block. The purpose of a freeze plug is to allow the engine to expand and contract without damaging the engine block. Freeze plugs are also used to seal off unused passages in the engine block.
Blue Devil is a product that claims to be able to fix a freeze plug. However, there is no guarantee that Blue Devil will actually fix a freeze plug. If you have a leaky freeze plug, it is best to replace it with a new one.