Spoiler tags are used to hide text unless hovered over. They are done by starting lines with '>! '.
Spoiler tags are one of the Stack Exchange extensions to markdown. They provide a way to hide text that should not be immediately seen, namely spoilers.
Spoilers are created by starting each line with
>! , similar to quote formatting (note: Each line must start with
>! , including blank lines)
>! 2 + 2 = 4
2 + 2 = 4
For a line break, use the
>! orange<br> >! apple
Copied from: http://meta.stackoverflow.com/tags/spoilers/info