What does a single tap on the lock screen do? The Swipe lock screen on my S4 running Jellybean and Kitkat has a number of actions. 


Swipe left for camera
Swipe right for other widgets 
Swipe down on the widget allows you to them click on the widget 
Swipe anywhere in the middle unlocks
A single tap on a widget does...  I have no idea.




What does this mode do? It shrinks the widget and Iightens the bottom half of the screen with no apparent purpose.
It's telling you that you will have to unlock the device before you can access a widget tap's action. 

Eg: If I tap on the clock widget on my lock screen, I will have to unlock the phone before it opens the clock screen. If I tap on a Google Now widget on my lock screen, I will have to unlock the phone before it opens Google Search/Now.

The dimming is trying to have you focus on the lock mechanism (where you swipe).