The climax is the moment the story has been building toward — the confrontation, choice, or revelation that resolves the central dramatic question. It's where the protagonist's wants, flaws, and the rising action collide and something is settled.
An earned climax pays off setup the reader half-forgot; an unearned one resolves the conflict through coincidence or a power the story never established. The difference is whether the ending feels inevitable in hindsight.
Example
Not 'a stranger arrives and fixes everything,' but 'the character finally does the thing she's avoided for 300 pages — and pays for it.'