Any updates on those tables would generate (and be stored in) rollback. Than applied after the table is released. That is what Oracle is saying

"If you use CONSISTENT=y and the volume of updates is large, the rollback segment usage will be large. In addition, the export of each table will be slower because the rollback segment must be scanned for uncommitted transactions."

If a consistent export is started then the export session has to read data as it was at the beginning of the export, regardless of any changes made. Therefore any changes made to the tables since the start of the export must be preserved in rollback segments until the export finishes.

An inconsistent export does a similar thing, but only for one table at a time

thanks