Fill dirt is a type of soil that is typically used to fill in low areas or to level the ground for construction purposes. It is made up of various types of soil, such as sand, clay, and topsoil, and may contain rocks, roots, and other debris. Fill dirt is often less expensive than other types of soil, such as topsoil, and is commonly used in large-scale construction projects. However, its quality and composition can vary significantly, which can affect its suitability for certain applications.