What do grayed-out app icons mean in iOS 7?

Question

Every couple days on my iPhone 4 running iOS 7, I notice an app icon is slightly grayed-out. Here is an example with the clock app:

When I open the app the icon goes back to normal immediately:

What does this mean? I can't find any documentation explaining it, and I can't determine a pattern for which apps exhibit it.

Answer 1

It's a glitch, where the grey overlay that appears on top of an app when you tap it is retained when you exit the app.

You can remove it by interacting with the app icon in some way, whether that be swiping between pages starting with your finger on the app, or simply tapping the icon.

^{It appears to be fixed in the dev build of iOS 7.1 :)}