AppLock is a new feature that Google introduced in their latest version of Android, Kit Kat 4.4. This allows you to lock your screen with a thumb print instead of typing in a password. It works by having the user enter their fingerprint into an application, which is run by your system (normally, you wouldn't need to use this feature). Once your fingerprint is detected, a number of things happen.
On your Android device, tap "Settings". 2nd, touch the icon for "Fingerprint". Now, select "set fingerprint password", and then use the type of password you want to use for AppLock. In most cases, you should not need to change this setting (unless you are using Google Now on your smartphone). However, it's up to you to set it if you like.
To use AppLock, you will need to configure two settings: one to lock the screen with a fingerprint and one to allow the user to unlock their phone with a PIN. This second option will also hide the fact that you are using AppLock. To do this, go to "Settings" and tap "Fingerprint." You will see a pop-up window asking you to enter a fingerprint, just as if you were to enter a PIN to unlock your phone. If you already have a lock set (and usually you do, since it's a security feature of Android), you will see a lock symbol next to the fingerprint field.