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.
Easy enough to find on Google.
Root dispersion is the maximum clock time difference that was ever observed between the local clock and the root clock.
Nothing to worry about.