Is used as a promotional method, to show the corresponding iOS app to a website in the websites header.
Read more about
apple-itunes-app on Apples Developer Guide to Webkit.
<meta name="apple-itunes-app" content="app-id=myAppStoreID, app-argument=myURL">
app-id is required and should include your app’s unique identifier.
app-argument is optional and helps your user to jump to the corresponding position to the websites position in your app.
In the WHATWG WikiUp to date: v2.3.5