A cold frame is a simple way to harden off transplants prior to moving them to the field. This provides a transition between the tender conditions of the greenhouse to the more variable outdoor environment.
The sash covered with glass or plastic is open in the day to prevent heat build up and closed at night to prevent frost damage.
A heated cold frame is called a hot bed and can be used on a small scale to start transplants without the expense of a greenhouse.
Cold frames are useful for out of season vegetable production, for holding transplants and for keeping potted seedlings for winter grafting.