Jephthah promises God that in return for help in slaughtering the people of Ammon he will offer up a human sacrifice and this is acceptable to God.

Jdg 11:30-31 And Jephthah vowed a vow unto the LORD, and said, If thou shalt without fail deliver the children of Ammon into mine hands,
Then it shall be, that whatsoever cometh forth of the doors of my house to meet me, when I return in peace from the children of Ammon, shall surely be the LORD's, and I will offer it up for a burnt offering.

God keeps his end of the bargain

Jdg 11:32-35 So Jephthah passed over unto the children of Ammon to fight against them; and the LORD delivered them into his hands.
And he smote them from Aroer, even till thou come to Minnith, even twenty cities, and unto the plain of the vineyards, with a very great slaughter. Thus the children of Ammon were subdued before the children of Israel.
And Jephthah came to Mizpeh unto his house, and, behold, his daughter came out to meet him with timbrels and with dances: and she was his only child; beside her he had neither son nor daughter.
And it came to pass, when he saw her, that he rent his clothes, and said, Alas, my daughter! thou hast brought me very low, and thou art one of them that trouble me: for I have opened my mouth unto the LORD, and I cannot go back.

Now Jephthah must sacrifice his own daughter.He must burn her to please God.

Jdg 11:39 And it came to pass at the end of two months, that she returned unto her father, who did with her according to his vow which he had vowed.