Hi Ed,
The rejected entries are objects or/and attributes from your LDAP which cannot be transformed (missing or/and corrupted values or characters) which cannot be converted.
This most happening when the LDAP is a Domino LDAP, however you can enable the Audit level from your LDAP source document (administration / LDAP) set your LDAP source and set it in verbose to get a xml file which contains the list of objects and some other info.
Once the Synchronization has run, from the interface click on Status / logs / select as components 'control center' - then it will appear a list of file, including the reject.xxx.xxxin xml format.
Hope that help a bit,
Nicolas.