The Literal Commandment
When brothers live together and one of them dies and has no son, the wife of the deceased shall not be married outside the family to a strange man. Her husband's brother shall go in to her and take her to himself as wife and perform the duty of a husband's brother to her. It shall be that the firstborn whom she bears shall assume the name of his dead brother, so that his name will not be blotted out from Israel. But if the man does not desire to take his brother's wife, then his brother's wife shall go up to the gate to the elders and say, 'My husband's brother refuses to establish a name for his brother in Israel; he is not willing to perform the duty of a husband's brother to me.' Then the elders of his city shall summon him and speak to him. And if he persists and says, 'I do not desire to take her,' then his brother's wife shall come to him in the sight of the elders, and pull his sandal off his foot and spit in his face; and she shall declare, 'Thus it is done to the man who does not build up his brother's house.' In Israel his name shall be called, 'The house of him whose sandal is removed.' (Deuteronomy 25:5-10)
The literal commandment is that a childless widow should not be married outside of her family... unless a certain ceremony takes place to free the brother and the widow from the levirite marriage.