After you build one or more packages, you have to make them available for download. This means you must copy the entire contents of the output directory (the directory named with -i on the ppackage command line) to a web server where it will be visible for download under a particular URL.
The root URL that contains the output directory, including the contents.xml file therein, is called the "host URL". This is the URL that you must use when referencing your package in your p3d file(s).
When the Panda3D plugin system downloads a package, it first downloads the contents.xml file at the root of the host URL, and it uses this file to determine which packages are defined at this host and whether any packages need to be redownloaded. So it is important to copy the entire contents of the output directory, and not to change any parts of it other than with the ppackage tool.
Although not strictly necessary, it is helpful to tell the package, at the time you build it, what its final host URL will be. You can do this by calling packager.setHost() at the top of the pdef file (before the first class definition), like this: