What is a shadow program committee?
The link that you provide explains it fairly clearly for that academic community. They have chosen to organize training in reviewing and conference organization by setting up a group that does the same work as the real program committee.  The shadow group's work, however, is done only for training purposes and does not affect the actual decisions on papers.  This is similar to a mock trial or moot court in the legal arena: a chance to do "the real thing" without doing real damage if one screws up.  It both gives practice and helps the main organizers identify people who are ready to be brought onto the real program committee.

This is not a widespread mechanism in the scientific, however, so other communities may not mean the same thing if they use the same words.