It's a rather flimsy door of thin sheet metal and glass that you often see on the outer side of a house door. In the winter, it provides a layer of dead air space that helps to insulate the house. In the summer, storm doors are often fitted with screens to provide more ventilation while keeping insects out. This is still common today, but was much more common before air conditioning was ubiquitous in the US. BTW, storm windows are also a thing, and are still common in the US for the same reasons.