Albert Barnes Commentary Jeremiah 23:16

"Thus saith Jehovah of hosts, Hearken not unto the words of the prophets that prophesy unto you: they teach you vanity; they speak a vision of their own heart, and not out of the mouth of Jehovah." — Jeremiah 23:16 (ASV)

How were the people to know the false prophets from the true? The former bring a message that fills with vain hopes, or speak a vision out of their own invention.