What is the keyguard in Android?
I googled it and couldn't find a definition. As usual the Android documentation was very "illuminating".
In a nutshell, it is your lockscreen.
PIN, pattern, face, password locks or the default lock (slide to unlock), but it is your lock screen.
Keyguard basically refers to the code that handles the unlocking of the phone. it's like the keypad lock on your nokia phone a few years back just with the utility on a touchscreen.
you can find more info it you look in android/app or com\android\internal\policy\impl
Good Luck !