The "viewerframe" directory is a default setting for many legacy Panasonic network cameras. The mode=motion parameter specifically refers to the MJPEG (Motion JPEG) stream mode, which allows the browser to display a live video feed rather than a static image. The vulnerability exists because:
Universal Plug and Play (UPnP) can automatically open ports on your router to make devices accessible from the web, often without you realizing it. inurl+viewerframe+mode+motion