YAAK stands for Yet Another Application Kit. It's a set of tools that help developing enterprise applications in python.
yaak.inject is a package from the YAAK toolkit that provides dependency injection to your applications. See this Martin Fowler's article for an explanation of dependency injection and its usefulness when developing enterprise application.
You should have easy_install (from setuptools or something equivalent) installed on your system.
To install the package, just type:
$ easy_install yaak.inject
You can also install the package from a source tarball. Decompress the source archive and type:
$ python setup.py install