That means the Transactions (Txn) is not committed, that means it did not do permanent change to database. Get the transaction number ( it will be in hex format) associated in that line and search for that number, you will see what are the queries or statement for that transaction.
Comments
I got this message from the following select statement which I see in the vertica.log.
2013-09-01 09:00:00.719 DistCall Dispatch:0x7f7198073210-b000000084bed4 [Txn] <INFO> Rollback Txn: b000000084bed4 'select count(distinct(xxx)) from yyy where aaa='zzz' ;'
I don't understand why rollback happened during select statement doing.
Best Regards.