Spoilers are experts from the real boom or story, that you may not know of yet. This is to tell you that if you have not read the book or story but plan to, but don't want to ruin it, to not read that fanfiction. The parts of the novel you may not know yet are written in the text. See fanfiction for more information on those.
"Hey Sarah, this now HP fic has spoilers in it for the new book. Should I read it?"
"Not if you want the boom to be a surprise, Judy".