This message indicates one or more loops through a table to return rows. Even the simplest access to a single table is an iteration, as shown here:
select * from publishers
QUERY PLAN FOR STATEMENT 1 (at line 1). STEP 1 The type of query is SELECT. FROM TABLE publishers Nested iteration. Table Scan. Forward scan. Positioning at start of table. Using I/O Size 2 Kbytes for data pages. With LRU Buffer Replacement Strategy for data pages.
For queries that perform nested-loop joins, access to each table is nested within the scan of the outer table.
See “Nested-Loop Joins” on page 128 for more information.