We can treat mobile as first class citizen. It provides more capatability than desktop.
In mobile, you can get current position by GPS sensor. You can get device rotation and motion via acceleromemter and gyroscope. You can accept camera photo upload via file input. Your reader can pin your web to their home screen.
In near future, you mobile web can also access the bluetooth, camera raw data, and push notification.
Imagine a web page and web utility that makes use of these capatability, which we never imagine them in desktop web. The mobile web is the first class client. That’s the meaning of Mobile First.