'Duty' in the Bible

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

But if the man like not to take his brother's wife, his brother's wife shall go up to the gate unto the elders, and say, My husband's brother refuseth to raise up unto his brother a name in Israel: he will not perform for me the duty of a husband's brother.