Some times in your code (or you have coded a new program) you will have a SELECT query which has ORDER BY clause with numbers in it, instead of actual column names.

SELECT
ACCOUNT_NO, — 1
ACCOUNT_HOLDER_NAME, — 2
ACCOUNT_OPEN_DATE, — 3
ACCOUNT_BALANCE, — 4
ACCOUNT_HOME_BRANCH, — 5
ACCOUNT_HOLDER_ADDRESS — 6
FROM ADDRESS
WHERE ACCOUNT_BALANCE < 1000
ORDER BY 4 DESC, 3
;

So order by in the above SQL statement is same as below (which uses column names)

ORDER BY ACCOUNT_BALANCE DESC, ACCOUNT_OPEN_DATE

So the numbers can be used in place of column names and these numbers are nothing but the place of the column in the SELECT statement.