The application is compatible with several dozen different router models, like Pirelli Discus, Eircom, Verizon FiOS or Alice AGPF. It is important to keep compatibility in mind (the complete list is on the website of the author), as it is necessary for the router to be on it for you to be able to decipher its password.
To use the application, all you have to do is open it, select the network that you want to connect to, and push "Start scan". Within about one or two minutes you will have to password on your screen. Once that happens, all you have to do is copy and paste it in the WiFi configuration on your device.
Router Keygen is a pretty useful tool that should never be used with bad intentions. It is true that with it you can "steal" the connection from other users, but it can also help you out in a legitimate way in more than one situation.