The FileSet Refresh action simply re-runs the process to find files specified in the FileSet. When the FileSet Define action runs it processes the FileSet at that time, but at a later point in the build it may be necessary to Refresh the FileSet as files may have changed (eg. more files added, files deleted, attributes changed, etc.)
On this page:
Refresh
FileSet
The fileset that is to be refresh to reflect what is currently present on disk.