This quotation is from the NKJV:
No weapon formed against you shall prosper,
And every tongue which rises against you in judgment
You shall condemn.
This is the heritage of the servants of the Lord,
And their righteousness is from Me,”
Says the Lord.
The chapter is a poem about Jerusalem, and this verse is promising protection against the enemies of Israel.
The word “prosper” sounds odd in this context, and the NKJV is simply preserving what was in the KJV. Modern translations (as opposed to revisions of the archaic KJV) make it much clearer. For example, the NIV has “no weapon forged against you will prevail” and the NET has “no weapon forged to be used against you will succeed”. It is a poetic way of saying that Israel would win any battle over Jerusalem.