A dirty read is an operation that might read data written to the database by transactions that are not yet committed. There is always a chance that any transaction will fail for some reason, and if this occurs, the system rolls back any updates it might have made. If those updates include data that has been read by another concurrently running transaction, then that read is said to have been dirty, because as far as the database is concerned, it never really existed. See SQL Request and Transaction Processing for details about transaction integrity and some of the problems that relaxing it can bring.