Albert Barnes Commentary Numbers 25:4

"And Jehovah said unto Moses, Take all the chiefs of the people, and hang them up unto Jehovah before the sun, that the fierce anger of Jehovah may turn away from Israel." — Numbers 25:4 (ASV)

Take - that is, assemble the chiefs of the people to you (compare the phrase “took men,” in Numbers 16:1). The offenders were to be first slain by the hands of “the judges of Israel” (Numbers 25:5), and afterward hung up “against the sun” (that is, publicly, openly; compare 2 Samuel 12:12) as an aggravation of their punishment. This would be done by impaling the body or fastening it to a cross. Compare Deuteronomy 21:23 note, and 2 Samuel 21:9.