This spoiler syntax is specific to Stack Exchange, and is not part of a standard markdown syntax.
Spoilers are used to hide text or images that would otherwise negatively impact another user's experience. They can be created using
>!This is hidden until your cursor hovers on top of it
This is hidden until your cursor hovers on top of it
Note: This is not part of standard markup spec and it won't be rendered in all the markdown parsers.