It's the table in the data source connection. It's as if you were writing the query using the Custom SQL option in the data source connection dialog.
For example in the SuperStore Excel data source you'd use [Orders$].
Typically you should reference your fields using '%1' style placeholders in your RAWSQL. For example, a field named [value] in your Tableau data source should be referenced as follows in a RAWSQL calculated field:
RAWSQL_BOOL( "%1 > 0", [value] )
Tableau will substitute the properly-formatted field name qualified by its table name when issuing the actual database query.
I hope this helps,