group by 1 means code example

Example: group by 1

SELECT account_id, open_emp_id
         ^^^^        ^^^^
          1           2

FROM account
GROUP BY 1;

In above query GROUP BY 1 refers to the first column in select statement which is account_id.

You also can specify in ORDER BY.

Note : The number in ORDER BY and GROUP BY always start with 1 not with 0.