Holika Dahan is celebrated by burning Holika, the devil. For many traditions in Hinduism, Holi celebrates the death of Holika in order to save Prahlad by God Vishnu , and thus Holi gets its name. In this ritual people burn the holika and prahalad is getting saved, here Prahlad is saved by that rope at the time of fire the holika. For complete details about Holika dahan see the comment.