Help me in solving GSQ81 problem

My issue

what is this line mean ‘GROUP BY 1’?

My code

/* Write a query to categorize the students based on the marks into grades and output the count of students in each grade. Give the Alias name for the CASE as 'Grades"
- Marks Less than 50 - C,
- Marks between 50 and 80 - B,
- Marks more than 80 - A */

    WHEN marks<50 THEN 'C'
    WHEN marks BETWEEN 50 AND 80 THEN 'B'
    WHEN marks>80 THEN 'A'
    ELSE 'NA'
END AS Grades,
COUNT(*) AS student_count
FROM marks

Learning course: SQL Intermediate
Problem Link: CodeChef: Practical coding for everyone


It means – to group by the first column of your result set regardless of what it’s called.
In above query GROUP BY 1 refers to the first column in select statement.