What is Spool Operator in SQL Server: SQL Server Performance Tuning

Spool operator scans the input and places a copy of each row in a hidden spool table that is stored in the tempdb database and existing only for the lifetime of the query. If the operator is rewound (for example, by a Nested Loops operator) but no rebinding is needed, the spooled data is used instead of re-scanning the input.
Table Spool is a physical operator.