I apologise for asking what seems like an easilly googlable question but I've tried, and even Microsoft's own article on W32tm doesn't seem to mention it.
I have recently been configuring time sync for a new DC on an existing domain (to replace an old DC holding the FSMO roles and set as the reliable time source)
On one member server I saw a Root Dispersion of about 4 seconds when running w32tm /query /status
which concerns me slightly, so I want to find out what it means.