Albert Barnes Commentary 2 Chronicles 35:3

"And he said unto the Levites that taught all Israel, that were holy unto Jehovah, Put the holy ark in the house which Solomon the son of David king of Israel did build; there shall no more be a burden upon your shoulders: now serve Jehovah your God, and his people Israel." — 2 Chronicles 35:3 (ASV)

Put the holy ark etc. - The ark of the covenant may have been temporarily removed from the holy of holies while Josiah carried out necessary repairs.

It shall not be a burden upon your shoulders - Josiah means that the task of removing and replacing the ark will no longer be your duty. The ark is to remain undisturbed in the holy of holies. You are to return to your former duties: serving God and instructing the people.