Explain why the property of glass in trapping heat is called greenhouse effect?

The property of glass is made use of in constructing green-houses used for protecting green plants in very cold climate. A greenhouse is a house-like structure made entirely of glass. The plants to be protected from the cold weather are placed inside this glass house. The glass roof and glass walls of the green-house allow the heat and light of the sun to pass through them and go inside it. They do not allow the inside heat to go out. In this way, heat gets trapped in the glass house due to which it becomes quite warm inside.