A frost plug is usually a cup shaped sheet metal or similar "plug" located within a piece of equipment and designed to be extruded from its location as and when the water in the equipment expands should it starts to freeze. If you look at an engine block you can usually see them located in a number of locations, however I'm not sure where you would see then located in a fire hydrant.
In some USA northern climates hydrants are provided with a small plug in the base. Thge plug is removed upon installation. After using a hydrant and the hydrant valve is closed, the water can slowly drain out through the hole. This prevents ice from forming during cold weather, which could expand and damage the hydrant. Sometimes referred to as "frostproof", that's how I made the connection to "frost plug".
