What is a tire iron and how is it used in relation to changing a flat tire?

A tire iron is a tool used to remove and tighten the lug nuts on a wheel. It is typically L-shaped and has a flat end for leverage. When changing a flat tire, the tire iron is used to loosen the lug nuts before lifting the car with a jack and removing the flat tire. After replacing the tire, the lug nuts are tightened using the tire iron to secure the wheel in place.