Rsync Accounts
These are used to upload logs for the release rescoring run as described in RescoreMassCheck.
http://svn.apache.org/repos/asf/spamassassin/trunk/masses/CORPUS_SUBMIT has information about how to request an account and get setup.
You also need a copy of the rsync tool. Most Linux distributions include this. Alternatively, pick it up at http://www.rsync.org/ .
Administrivia
The server-side (adding accounts etc.) is discussed at RsyncConfig.