A spoiler is something that gives away information about future W.I.T.C.H. issues that haven't been released yet. In the comic scan thread you can see what the latest issue is.
This forum is spoilerfree.
Posting spoilers violates board rules, and fitting action will be taken if you do so. This can vary from a warning till a long term ban, depending on the frequency and amount of spoilers.
Also give other users time to read the latest issue. This usually takes a couple of days, and in that time we appreciate it if you don't post spoilers from the latest issue on the board.