a subquery is a SQL query inside another

Also called nested queries. You can use these to resolve one-to-many relations, eg find all the books from a specific publisher

SELECT "title" FROM "books"
WHERE "publisher_id" = (
  SELECT "id" FROM "publishers"
  WHERE "publisher" = 'the publisher I care about'
);

The bit in brackets () is the subquery. You could also use the IN keyword if your subquery returns more than one thing.

You can also use subqueries for many-to-many relations via an intermediate table.

SELECT "name" FROM "authors"
WHERE "id" = (
  SELECT "author_id" FROM "authored"
  WHERE "book_id" = (
    SELECT "id" FROM "books"
    WHERE "title" = 'some title'
  )
);

But this is starting to get really messy and hard to follow. You might prefer to use joins