The short answer is that decadence, for Nietzsche, is being drawn to what is bad for you.
Acting effectively requires self-confidence, great passion to achieve one’s aim, and unity of purpose. To be very successful, and certainly to achieve anything truly great, all of one’s abilities and all aspects of one’s personality must be devoted to achieving one’s aim.
This requires self-mastery, by which Nietzsche means the ability to cultivate one’s drives, desires, and abilities in ways that maximize their contribution to one’s project. Self-mastery is not achieved by conscious deliberation alone; it is an “instinctive” ability to do what is good for you. Faced with a choice, one with self-mastery will identify the best course of action without needing to deliberate.
He guesses what remedies avail against what is harmful; he exploits bad accidents to his advantage; what does not kill him makes him stronger. He collects instinctively from everything he sees, hears, lives through, his sum: he is a principle of selection, he leaves much behind. He is always in his own company, whether he associates with books, human beings, or landscapes: he honors by choosing, by admitting, by trusting. (Ecce Homo, “Why I Am So Wise” §2.)
The decadent, on the other hand, chooses what is bad for him, again in a largely non-deliberative way. A decadent or “corrupt” person instinctively seeks out that which harms him.