Probably your Xampp Server is part of a LAN and sits behind a router?! You must configure this router to pass all requests from Port 80 (and Port 443 for https) to the Xampp Server (this is usually called "Portforwarding", see the manual of your router) and in order to reach the Xampp Server from outside netword, you must enter the external IP of the router (which is provided by the Internet Provider) in your browser (iPad etc.) and which mostly is changing every day. It depends on your internet connection.
If you have a typical connection with dynamically changing IPs (what is very common), you may use a Dynamic DNS Service to assign a Domain Name to your router, which also changes the IP concurrently. This must be supported by your router as well (many routers support some of the "big" DNS Services like www.dyndns.org