en_tn_lite_do_not_use/rut/03/13.md

if he will perform for you the duty of a kinsman

"If he will redeem you" or "if he will marry you." A dead man's closest male relative was expected to marry the widow, and their first son would have the dead man's family name.

by the life of Yahweh

"as surely as Yahweh lives." Boaz used this vow to assure Ruth that he would keep his promise.