A freeze plug is a round piece of flat metal (steel or bronze), about an inch or so in diameter, that has about 1/4" lip all the way around it. It is pressed into a hole in the side of the block/head that was originally there to remove the core sand. Its secondary purpose is to pop when your coolant starts to freeze in your block because there isn't enough antifreeze in the mix.
The cost should only be a buck or two. Getting the old one out and the new one in (with the engine/transmission still in the car) is another story ! If you can get to it, drill a hole through the center big enough for a slide hammer hook and pop it out. Press the new on in using a hammer and a block of wood.
