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)

For example:

>! 2 + 2 = 4


2 + 2 = 4

For a line break, use the br tag:

>! orange<br>
>! apple



Copied from: http://meta.stackoverflow.com/tags/spoilers/info