A rotary laser is a tool used in construction and surveying to create a level reference line over a large area. It projects a rotating laser beam that can be detected by a receiver, allowing workers to establish a precise horizontal or vertical plane. This makes it easier to align structures, grade land, and ensure accurate measurements. These lasers are often used in conjunction with other equipment, such as tripods and laser receivers, to enhance their functionality. They are essential for tasks like building foundations, landscaping, and installing fences, providing efficiency and accuracy in various projects.