True to its name, a cold frame is a transparent structure that protects plants from cold temperatures while allowing sunlight to pass through, via the University of Missouri Extension. Usually, the temperature inside the cold frame is only a few degrees warmer than the outside, which often makes all the difference. Cold frames may look different depending on the available materials and overall handyman skills, but the principle remains the same. While cold frames were traditionally used to harden seedlings, they have found multiple uses that have been revolutionary to gardeners over time.
Other uses of cold frames include; extending the growing season of some plants past frost, overwintering plants, and starting cold hardy species way earlier than they can be grown on bare soil. However, there are a couple of things that you have to realize before constructing a cold frame. First, not every plant tolerates cold temperatures; some cannot survive even when protected in a cold frame. Also, the materials you use will determine how much protection the plants will get.