A subresource is basically additional content that a page includes via a URL (e.g. script, iframe, images). The problem underlying this is that any page on the

A subresource is basically additional content that a page includes via a URL (e.g. script, iframe, images). The problem underlying this is that any page on the web can currently probe and then attack devices and servers on your internal network or local host. So, the proposal is to block that by preventing remote websites from navigating to or including sub-resources from anything on your internal network or local host (unless some explicit opt-in or exception is configured).