All right, to give you an idea of what subqueries are, consider the following problem: we want to find cities which have the same rating as Paris.

With the knowledge you have now, you would first need to check the rating for Paris:

SELECT rating
FROM city
WHERE NAME = 'Paris';

Then you would need to write down the result of the above query somewhere in your notebook (the rating is 5, by the way) and then construct a new query:

SELECT name
FROM city
WHERE rating = 5;

Subqueries have been introduced to help you with such examples. They are "queries inside queries" and they are always put in parentheses. Take a look:

SELECT name
FROM city
WHERE rating = (
  SELECT rating
  FROM city
  WHERE name = 'Paris'
);

The database will first check the subquery (in the parentheses), then return the result of the query (in this case, the number 5) in place of the subquery and then check the final query.

In this particular example, you must write the subquery in such a way that it returns precisely one value (one column of one row) so that it matches the equation rating = X. It wouldn't make much sense to put a whole table there, would it?