A query can contain one or several subquery. A subquery is also known as query block.
SELECT e.ename, e.sal
FROM (SELECT *
FROM emp e
WHERE e.sal > 300
AND e.deptno IN (SELECT d.deptno
FROM dept d
WHERE d.dname IN ('SALES', 'ACCOUNTING'))) e
This query contain 3 query block :
SELECT d.deptno
FROM dept d
WHERE d.dname IN ('SALES', 'ACCOUNTING')
SELECT *
FROM emp e
WHERE e.sal > 300
AND e.deptno IN ('last query block')
SELECT e.ename, e.sal
FROM ('second query block')