What goes on the inside of fireplace walls?

I recently had my fireplace cleaned and the guy told me that there is supposed to be another layer over the metal on the inside of the fireplace. he said I can buy it in a sheet from most home repair store, most places have it in a brick imitation style, and it has to be cut with a diamond blade. he said that it will definitely extend the life of my fireplace and that he doesn't know why it wasn't in there to begin with does anybody know what its called? im looking for some on the internet but i don't know what its called. thanks.