Please help me analyze the explain plan:

Here is the explain plan:

SELECT STATEMENT COST=15781
VIEW ORDER_DATA
UNION-ALL
TABLE ACCESS BY INDEX ROWID INTRA_DAY_ORDER_BOOK
INDEX FULL SCAN INTRA_DAY_ORDER_BOOK_IDX2
FILTER
TABLE ACCESS BY INDEX ROWID INBOUND_ORDER
INDEX RANGE SCAN INBOUND_ORDER_IDX2
INDEX RANGE SCAN INTRA_DAY_ORDER_BOOK_IDX3

Q: WHAT is a INDEX FULL SCAN and how can I avoid it ?