A 'spoiler' is something that'll give away crucial bits of the plot if somebody reads it. Marking spoilers is just a courtesy thing for people who haven't finished the game/novel/movie, whatever it is you're discussing at any given time.

To mark a spoiler, just add [sp</>oiler] YOUR TEXT HERE [/spoiler] tags around the text which contains the 'spoiler'. Then, to view a spoiler, just highlight it with your mouse.

(SPOILER)See? Not really that hard at all, is it?