Preprocessor directives are similar to a WikiLink in form, except they contain spaces and parameters. The general form is:
[[directive param="value" param="value"]]
This gets expanded before the rest of the page is processed, and can be used to transform the page in various ways.
Note that if a preprocessor directive has no parameters, a space still must be put after its name, to avoid confusion with a WikiLink. For example:
[[pagecount ]]