Sentences that have not yet been validated are dimmed out like this.. Any sentence becomes color=gray>unchecked whenever you modify it.
By default, the CLChecker will check any sentence you modify as soon as you are done modifying the sentence. (It does this by waiting until you move the cursor outside the sentence.)
The Check All action validates every sentence in the document.
All sentence validation is done in the background, so you can continue to edit the document while validation is taking place.