I'm pretty sure most would know what spoilers are. You put the tags on your post, and there's a button on your post that hides whatever is in the spoiler.
Makes it great for people posting ABing lists so people who are viewing the thread won't have to scroll down for hours finding the thing they want.
It can also hide stuff that the average person wouldn't want to see.
So yea...I would appreciate if this got implemented (which I'm pretty sure is possible).