From Just Solve the File Format Problem
A Meta-Package File is a variant of the Package File that is used in Mac OS X when it is necessary to have an installation archive which can contain multiple software packages for distribution. Clicking on it on an OS X system will launch the Installer package to extract and install the software.
The meta-package may contain individual files or package files which are entire installable packages in their own right.
OS X packages are a form of the xar archive format.