When you start the app you'll see a list of all of the WiFi networks near you, which will appear in green or red depending on whether or not the app has been able to identify the password. When you tap on one of the networks that appears in green you can instantly see the password and, with a single touch, copy it to your clipboard.
In this way you can also find out if the password that it shows you (which will always be the password that the router uses by default) will allow you to easily access the network.
WiFi Keys is one of those apps that you shouldn't use if you're not sure what you're doing. Though it is easy to manage and it can help you solve problems with your own connection, the consequences of using it with unknown connections could be very negative.