What does that mean?

Querying hybrid table without a prefix uses a timeboundary at the broker level to decide how much data to be queried from offline vs realtime table. The determination of timeboundary is based on the offline table's segment end times and the ingestion frequency as defined in the table config https://dev.startree.ai/docs/pinot/concepts/time-boundary#how-is-the-time-boundary-determined