The Khandava forest (Khandavaprastha) was the home of the snakes, and it was the piece of land that Dhritarashtra gave to the Pandavas.
It had to be cleared and made arable. King Svetaketu (sometimes called Svetaki) made a twelve year fire sacrifice. In the fire sacrifice clarified butter is offered to the fire (Agni).
After the twelve years Agni absolutely needed something else to eat (burn), he wanted to eat (burn) the Khandava forest. Arjuna and Krishna made this possible.
Now the Pandavas could build their capital, Indraprastha, on it.