This is the end of the sentence that begins with the words "The Holy Spirit spoke" in verse 25 and that contains quotations within quotations. You can translate one of the inner quotations as an indirect quotation, or you can translate two of the inner quotations as indirect quotations. Alternate translation: "The Spirit told Isaiah to go tell them that they will hear but will not understand and they will see but they will not know" (See: [[rc://en/ta/man/jit/figs-quotesinquotes]])
Both of these phrases mean basically the same thing. They emphasize that the Jewish people will not understand God's plan. (See: [[rc://en/ta/man/jit/figs-parallelism]])