Please explain how WITH Clause working in a sql query?

An inline view or a query uses subquery in it.
Subquery execution done more efficiently rather by using the query repetadly.
On case by case the performance varies with WITH clause in a sql query.

Below example shows how to reduce repetition of query and simplify complex SQL statements.

SELECT e.ename AS employee_name,
       dc.d_count AS emp_dept_count
FROM   emp e,
       (SELECT deptid, COUNT(*) AS d_count
        FROM   emp
        GROUP BY deptid) dc
WHERE  e.deptid = dc.deptid;

Query will be like as below by using a WITH clause.

WITH d_count AS (
  SELECT deptid, COUNT(*) AS dept_count
  FROM   emp
  GROUP BY deptid)
SELECT e.ename AS employee_name,
       dc.dept_count AS emp_dept_count
FROM   emp e,
       d_count dc
WHERE  e.deptid = dc.deptid;

This way we can make multiple references to the subquery more efficient.

Sometime,WITH clause can be used for a subquery to use as a global temporary table or the query to be use as inline query.