First of all It is a co-related sub query . Now as you can observe , the NOT EXISTS clause is used . So the outer query will print a tuple if and only if the inner query returns nothing .
That is if at least one person who is of age less than 22 , and has rating greater than or equal to the rating of the sailor of the outer query , then the outer query shall not print anything .
So only if there is no such sailor whose age is less than 22 , and has rating greater than or equal to some sailor of the outer query , only then a sailor name shall be printed .