The apple-mobile-web-app-capable metatag defines wether a website/webapp which was added to the homescreen can run in fullscreen or not.
In favor of Progressive Web Apps (PWAs), this does not work anymore and is therefore marked as Unsupported.
Read more about Apple's Webapps at Apple's official documentation (opens in a new tab).
<meta name="apple-mobile-web-app-capable" content="yes">
content can also be
false, but the meta-tag is obsolete then.