Moses spake unto the heads of the tribes— i.e. the representatives of the people assembled together, Deuteronomy 33:5. 1 Chronicles 28:1. It is very likely that some case had been propounded to Moses about vows; concerning which, by the Lord's command, he here gives them such rules as might direct them for the future.
Related Commentaries of Numbers 30:1
Numbers 30:1
1 And Moses spake unto the heads of the tribes concerning the children of Israel, saying, This is the thing which the LORD hath commanded.