its like a little plug that is made usualy of aluminum that has a little bit larger outer end on one side so then you push it in a hole it will stay tight and in place. If you ever need to pull one on a tranny (specificaly for a speedo housing) be very very careful.
There are frezze plugs all over the block and head. This is incase the collant starts to freeze and expand it will push outr these plugs verses cracking the block. There are like 1" to 2" in diameter and pressed in.
Also refered to as "core plugs" or "soft plugs" They fill holes in castings that are put there to allow casting material a way out or to plug "galleys". The term "freeze plug" comes from the hope that frozen water in an ill-prepared cooling system will force them out before the block cracks from the pressure....... :whoops: