French doesn't let you skip the article the way English does with a sentence like "I'm eating bread." It inserts a partitive article — du, de la, de l', or des — to mean "some" of an unspecified amount, agreeing with the noun's gender and number. That article doesn't survive everywhere, though: negate the sentence, or put a quantity word in front of the noun, and du, de la, and des all collapse down to a bare de.
Du, de la, des mean "some" in a plain affirmative sentence, matching the noun's gender and number. De (or d') replaces all of them after a negation or a word expressing quantity.
Affirmative sentences -- "some"
After a negation or a quantity word
Je ne bois jamais de café le soir, mais ce n'est pas du vrai café de toute façon.
I never drink coffee in the evening, but it isn't real coffee anyway.
The reduction to de only happens when a negated verb like boire is saying the noun doesn't exist in the sentence at all. Negating être to make an identity claim ("it isn't [x]") doesn't trigger the same collapse -- du/de la/des survive after ce n'est pas.
Take the quiz and see if you can tell when du, de la, and des collapse to de.