It does three main things:
Replace the old drawer icon: with the new drawer icon from L:
Change the height of the actionbar on AppCompat and ActionBarSherlock apps to the L size
Changes the back icons to L style ones
It also provides a GUI to switch from dark to light icons on a per-app basis, as well as turn off the icon changing for apps you don't want theming
Essentially, it fills the gaps of the theme.
APK is also attached