The hearth is the flat surface found at the base of all traditional open wood burning fireplaces.
We have an article that covers fireplace hearths in more detail here, but a hearth will be made of a fire-resistant material such as concrete, granite, tiles or stone.
A hearth is required for all wood burning fireplaces (including stoves) for safety reasons, as it provides a barrier between the heat of the fire and the floor of a home.
For open wood burning fireplaces, the fire is traditionally built on the hearth or within a fireplace grate. The hearth must therefore be able to cope with the high temperatures of such a fire.
Wood burning stoves work by absorbing heat from a fire located inside and transferring it out into the room in a more efficient manner compared to open wood fires. This means that the body of a wood stove gets extremely hot and so a suitable hearth is also required for safety purposes.