A rotary laser level 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 users to establish a level plane for various tasks, such as grading, aligning, and installing fixtures. These devices are often used for outdoor projects due to their ability to cover long distances and maintain accuracy. Rotary laser levels come in different types, including manual and self-leveling models, making them versatile for both professional and DIY applications.