'Duty' in the Bible

'When brethren dwell together, and one of them hath died, and hath no son, the wife of the dead is not without to a strange man; her husband's brother doth go in unto her, and hath taken her to him for a wife, and doth perform the duty of her husband's brother;

'And if the man doth not delight to take his brother's wife, then hath his brother's wife gone up to the gate, unto the elders, and said, My husband's brother is refusing to raise up to his brother a name in Israel; he hath not been willing to perform the duty of my husband's brother;