There are several different technologies that use the term file shadowing, but the intent of file shadowing within these technologies is to provide an exact copy (or mirror of a set) of data.
For file shadowing to be effective, the shadow needs to exist in a separate physical location than the original data. Depending on the reasons behind the shadow operation, this location may be as close as a second harddrive in the same chassis or as far away as the other side of the globe.
References
File shadowing Wikipedia(Text) CC BY-SA