'Duty' in the Bible

If brothers dwell together, and one of them die, and have no son, the wife of the dead shall not be married outside to a stranger: her husband's brother shall go in to her, and take her to him as wife, and perform the duty of a husband's brother to her.

If the man doesn't want 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 raise up to his brother a name in Israel; he will not perform the duty of a husband's brother to me."