Complex sentence: A variety of block heater called a "freeze plug heater" can be installed, replacing one of the freeze (core) plugs, to warm engine coolant (and therefore the engine) before start up.
Split into simpler sentences: A variety of block heater called a "freeze plug heater" can be installed, replacing one of the freeze (core) plugs. The electrical heater is used to warm the engine coolant (and therefore the engine) before start up.