A shoe tree is a device approximating the shape of a foot that is placed inside a shoe to preserve its shape, stop it from developing creases and thereby extend the life of the shoe.
Higher quality shoe trees are made from solid wood, usually cedar which helps control odor and absorb moisture. Wooden shoe trees are often made with two or three pieces of solid wood with a solid metal stem inserted between the heel piece and the single or double toe piece/s which have a spring action so the trees fit more snugly into the shoes. They often have handles or brass knobs at the heel piece for the fingers to grasp and pull out the trees from the shoe when removing them.
Shoe trees may also be made of plastic, with or without a coiled metal spring stem; these are typically cheaper, lighter, and are better suited for travelling.