When there are inserts, updates, or deletes to documents in your base table, the DML queue stores the requests for documents waiting to be indexed. When you synchronize the index with CTX_DDL.SYNC_INDEX, requests are removed from this queue.
Pending DML requests can be queried with the CTX_PENDING and CTX_USER_PENDING views.
DML errors can be queried with the CTX_INDEX_ERRORS or CTX_USER_INDEX_ERRORS view.
Oracle Text Reference for more information about these views