The joke is very simple. You choose one of the WiFi networks within range of the device, select it, and then you'll begin to see messages saying you're hacking into the network. It's a lie. The network is not being hacked at all, but your friends might fall for it.
After a few seconds of intense hacking, Wifi Hacker Prank shows you a random number that's supposedly the password, but it's not at all. The number definitely has nothing to do with the actual password.
Wifi Hacker Prank is a slightly lame app that's only good for pranking your friends.
Requires Android 2.3 or higher