Sets the directory to cache "prepared" templates in, the directory should be writable for PHP.
The "prepared" template contains an internal representation of template structure: essentially a serialized array of $_blocks, $_blockVariables, $_children and $_functions, may also contain $_triggers. This allows to bypass expensive calls to HTML_Template_Sigma::_buildBlockVariables() and especially HTML_Template_Sigma::_buildBlocks() when reading the "prepared" template instead of the "source" one.
The files in this cache do not have any TTL and are regenerated when the source templates change.