The hearth of a fireplace is basically the floor that your heat source sits on. As this is directly next to the heat source, you need to ensure your base is fire-resistant and can withstand the heat for all solid fuel burning. A hearth isn’t required for an electric fire.
Usually brick, stone, marble, or cement is used to create a hearth. Not all stone and marble are suitable, and often need to be specially treated for use in a hearth.