Foiling is a printing process using heat to fuse a fine layer of foil (often gold or metallic) onto card.
A magnesium die of the image has first to be produced, replicating the image or copy in reverse. This die is then mounted on a press that has a heated bed and which transfers heat throughout the die. When the heat is correct, the foil is placed above the card and the die brought down to stamp the foil onto the chosen card. The result is a slightly indented, but very metallic looking print.
Foiling is not suitable where very fine detail is needed to show in a design.