In the standard XMPP protocol, a client must ask the server for the user’s contact list (roster) each time it logs in. If (like certain people) you have a lot of contacts, then it could be a significant saving in bandwidth and processing time to be able to skip this roster download.
Thankfully that is exactly what roster versioning allows. If your server sees that your client already has an up to date version, it will simply let it know that, or send a small list of changes if there have been any. This saving is especially useful on low-bandwidth or pay-as-you-go data connections.
To the best of our knowledge Prosody is currently the only server to support roster versioning, and Gajim the only client. It would be great to see other clients and servers now follow the lead the two have taken in proving protocol, and improve login times for all Jabber/XMPP users.