Core plugs is fitted in the round holes along the engine block. They keep the antifreeze inside, and I suppose they're meant to pop out if the antifreeze should freeze (normally below -34 degrees celsius).Changing them is quite fiddly at least when the engine is in the car. The biggest problem is finding room for your hands and tools.
To remove a core plug take a punch and a hammer, and knock on one side of the coreplug, until it turns in the hole. Then grab hold of the other side with a pair of pliers. Twist and pull to get it out. Be extremly careful not to knock the plug into the engine block. You might not get it back out, and it might get stuck in