SleepyKitty is an application that shuts down the Android phone after 3 failed unlock attempts. To shut down the phone, it requires root permissions.
If you encrypt your phone, you need a strong password. You do not want this password to be your screen password because unlocking your phone would take very long, due to the strength of the chosen password. Hence, you use a short PIN as your screen password. However, now someone could brute force this PIN when your phone is stolen. SleepyKitty solves this problem. After the weak screen password is entered wrong three times in a row, it simply shuts down the phone. Now, the data is back to its encrypted state and can only be accessed with the strong disk encryption password.
When you run SleepyKitty, it will attempt to gain root permissions. This is because shutting down your phone requires root permissions. If you want the application to work, you will have to give it root permissions and check the checkbox that says "remember my choice" so SleepyKitty can elevate to root when it wants to initiate the shutdown. The second thing SleepyKitty will do is install itself as Device Administrator, in order to monitor the lock screen.
Enjoy!