A French Top is typically a 24" x 24" area on the range top that is made from a heavy flat smooth steel plate with a high powered gas burner located in the center, but underneath the surface. (similar to the way a teppan-yaki griddle is made).
There are usually rings, there could be 1 to 3 additional ring plates that, if removed, would enlarge the central opening. The idea of the French Top is that when all the plates are in place, you have a flat surface to move cookware around on, with high heat in the center gradually cooling off as you move further away.
There are many great manufactures that offer a French Top, including Wolf, La Cornue, The French top/plaque is pure cast iron. This means it must be seasoned. To season brush mineral or vegetable oil to the entire area of the plaque, turn on the plaque, and let it bake in for 30 minutes. This should be the maintenance ritual practiced on the plaque weekly if the plaque is being used daily.
Also remember that iron turns. Your plaque will not remain silver in tone, but rather a brownish/blackish patina will appear. Enjoy the experience.