A cold frame is a simple structure used in gardening to protect plants from cold weather. It typically consists of a transparent lid, often made of glass or plastic, that allows sunlight to enter while trapping heat inside. This helps create a warmer environment for seedlings and young plants, extending the growing season. Cold frames can be placed directly on the ground and are often used to start seeds early in the spring or to harden off plants before transplanting them outdoors. They are an effective way to grow vegetables and flowers in cooler climates, making them a valuable tool for gardeners.