ONLINE Mode Reindex:
In ONLINE mode, the new index is built while the old index is accessible to reads and writes. Any update on the old index will also get applied to the new index. An antimatter column is used to track possible conflicts between the updates and the rebuild (ie. delete of a row which was not yet copied). When the process is completed, the table is locked for a brief period and the new index replaces the old index.

OFFLINE Mode Reindex:
In OFFLINE mode, the table is locked upfront for any read or write, and then the new index gets built from the old index while holding a lock on the table. No read or write operation is permitted on the table while the index is being rebuilt. Only when the operation is done is the lock on the table released and reads and writes are allowed again.