The hanging of leaves near the entrance of the house is popularly known as toran or thoranam.
It is common practice for Indians to decorate the main door of the house, temples or areas where rituals are being performed to be decorated with leaves. What many see as just a tradition actually has scientific backing.
The garland is made with fresh green mango leaves because they are able to absorb carbon dioxide and release oxygen. This helps in keeping the surrounding atmosphere clean and fresh.
It is considered extremely auspicious as it is believed that it keeps the evil spirits and negative energy at a bay. It is also believed that tying the leaves can make the house owner's desires come true. This is why mango leaves are used or ashoka leaves.
The significance of hanging leaves is also mentioned in various Puranas and in the Bhagavad Gita.