Per Success Criterion 2.4.1 (Bypass Blocks), frames and iframes should be titled (using the title attribute) so that the purpose of the frame is described, providing the user with information to assist them in their choice to bypass the frame or not.
If a title is used, it should describe the purpose of the frame. The notice emitted is to remind authors of this responsibility. For avoidance of doubt, it should describe the purpose of the frame, not of the document. The document's own title element holds the latter responsibility.