How are you connected to the internet? Through a router? Not so sure how you'd do this whilst tethered, that'd be messy, but this is how you'd do it over Wifi and a router:
Think of it like this. You get given one IP address by your service provider, and that is known as your external IP and is your router's IP address. Now, since your router has taken that address, none of your other devices can use it. So, how do you solve that? Your router assigns each device a local/internal IP address. These always start with 192.168.X.X . Lets say your android device has been given 192.168.0.5, as an example.
Now, port forwarding comes in when you try to play a game over a specific port. Say your game uses port 1234. Information about your game will be sent to your router via your external IP, but... where does your router send it? How can your router tell which internal IP address it should use? If the router guesses wrong then you can't connect on the device you want to.
So, you go into your router's settings and tell it "always send traffic (i.e. 'forward') arriving on port 1234 to 192.168.0.5". And that's it, you can now connect. Your friend will have to do the same with his router so he can connect too. The same is true regardless if your friend lives down the road or in another country.
You will need to look into figuring out which local IP address your android phone gets assigned, and how to configure port forwarding on your particular brand of router.
