He's putting the example data into CTEs... It's the same as if he showed you the solution like this:

-- Sample Data

DECLARE @charges TABLE ([ChargeAmount] money,[PeopleID] int);

DECLARE @payments TABLE ([Amount] money,[PeopleID] int);

INSERT @charges

SELECT 100.00, 54215 UNION ALL

SELECT 125.00, 53252 UNION ALL

SELECT 110.00, 63225 UNION ALL

SELECT 10.00, 54215 UNION ALL

SELECT 10.00, 53252 UNION ALL

SELECT 15.00, 63225;

INSERT @payments

SELECT 75.00, 54215 UNION ALL

SELECT 25.00, 53252 UNION ALL

SELECT 50.00, 63225 UNION ALL

SELECT 10.00, 54215 UNION ALL

SELECT 25.00, 53252 UNION ALL

SELECT 25.00, 63225;

-- Solution

WITH

NetCharges

AS

(

SELECT SUM([ChargeAmount]) As TotalCharges ,[PeopleID] FROM @Charges GROUP BY [PeopleID]

),

NetPayments

AS

(

SELECT SUM([Amount]) As TotalPayments ,[PeopleID] FROM @Payments GROUP BY [PeopleID]

)

SELECT c.peopleID,

TotalCharges,

TotalPayments,

TotalCharges - TotalPayments as [Balance]

FROM NetCharges c

inner join NetPayments p

on c.peopleid = p.peopleid;