Jamieson, Fausset and Brown on Deuteronomy 25:5-10

5-10. the wife of the dead shall not marry without unto a stranger: her husband's brother ... shall take her to him to wife--This usage existed before the age of Moses (Ge 38:8). But the Mosaic law rendered the custom obligatory (Mt 22:25) on younger brothers, or the nearest kinsman, to marry the widow (Ru 4:4), by associating the natural desire of perpetuating a brother's name with the preservation of property in the Hebrew families and tribes. If the younger brother declined to comply with the law, the widow brought her claim before the authorities of the place at a public assembly (the gate of the city); and he having declared his refusal, she was ordered to loose the thong of his shoe--a sign of degradation--following up that act by spitting on the ground--the strongest expression of ignominy and contempt among Eastern people. The shoe was kept by the magistrate as an evidence of the transaction, and the parties separated.