Bhagavad Gita, Chapter 4, Verse 22

Content with what comes to him without effort, unaffected by the pairs of opposites, free from envy, even-minded in success and failure, though acting, he is not bound.

Success and failure stop having the same grip on someone who's genuinely made peace with either outcome.

This describes real equanimity, not indifference. Whatever actually comes, comes, and that steadiness through both wins and losses is what keeps a person from getting entangled in either one.