This corrects the `mkdir` command used by rsync on Windows to make sure the destination directory exists before starting to sync. The old form was correct on Linux but not on Windows, and it was just a coincidence that the `-p` argument appeared to be work.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.