At its core, is a specific display state or URL parameter used primarily by network cameras (like those from Panasonic, Sony, or Axis) to deliver a live video stream through a web browser.
Viewerframe modes that rely on ActiveX only work in Internet Explorer or "IE Mode" in Microsoft Edge. They will typically fail in Chrome or Firefox.
Most cameras require a username and password. If your viewerframe isn't loading, ensure you are logged into the camera's root IP first.