After the judgment of Urim... — See Exodus 28:30, and Note.
At his word... — i.e., Joshua and the children of Israel were to abide by the decision of the high priest, which was obtained by means of Urim and Thummim.
Related Commentaries of Numbers 27:21
Numbers 27:21
21 And he shall stand before Eleazar the priest, who shall ask counsel for him after the judgment of Urim before the LORD: at his word shall they go out, and at his word they shall come in, both he, and all the children of Israel with him, even all the congregation.