What does a single tap on the lock screen do?
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).