Expanded metal is a type of sheet metal which has been cut and stretched to form a regular pattern (often diamond-shaped) of metal mesh-like material. It is commonly used for fences and grates, and as metallic lath to support plaster or stucco.
Expanded metal is frequently used to make fences, walkways, and grates, as the material is very durable and strong, unlike lighter and less expensive wire mesh. The many small openings in the material allow flow through of air, water, and light, while still providing a mechanical barrier to larger objects. Another advantage to using expanded metal as opposed to plain sheet metal is that the exposed edges of the expanded metal provide more traction, which has led to its use in catwalks or drainage covers.
(Expanded metal, Wikipedia)