I think it referres to information that is reaveled about an 아니메 또는 a Character etc that other people watching the show may not yet know. Some people don't want to know what happens, befor they get to see it for themselves, because it can Spoil the surprise. that is why it is called a Spoiler:)
A "spoiler" is information that may "spoil" a book, show, 또는 movie for someone. For example if someone wrote "I can't believe _________ died at the end of ________!" That would be a spoiler.
Sometimes with such questions I wonder why someone don't 검색 in 구글 when he something don't know... I just tick and knows everything Google, don't bite! Are people are so lazy that they must ask for something other? I will understood if it were something difficult to find, but this? Other 질문 that are really hard to find the answer are 로스트 in 질문 like this one...
