A spoiler is a piece of information that, when released too early, might "spoil" someone's enjoyment of said thing in question. For example, information about who-done-it spoils a mystery; information about who-ends-up-with-whom could spoil a romantic story; or who wins the battle and who dies could spoil an epic about a war between good and evil. Anything about plot twists or surprises could be a spoiler.