DEVOTED THING (NIV Dictionary) That which is set apart to the Lord. A sacrifice or offering is a voluntary gift from the owner and can be recalled at any time before the ceremony, but not a devoted thing. Achan's sin at Jericho (Josh 6:17--19) was considered far more serious than mere stealing, for he had taken something devoted. Nations, cities, or men who were "devoted" were to be utterly destroyed---e. g., the Amalekites (1 Sam 15).
Lev 27:28(NIV Study Bible) devotes to the LORD. See NIV text note. Devoting something was far more serious than dedicating it to sacred use. The devoted thing became totally the Lord's. Achan's sin was the greater because he stole what had been devoted to the Lord(Jos 7:11). Persons devoted to destruction were usually the captives in the wars of Cannan(cf. 1Sa 15:3, 18).