What is choice_set in this Django app tutorial?
You created a foreign key on Choice which relates each one to a Question.
So, each Choice explicitly has a question field, which you declared in the model.
Django's ORM follows the relationship backwards from Question too, automatically generating a field on each instance called foo_set where Foo is the model with a ForeignKey field to that model.
choice_set is a RelatedManager which can create querysets of Choice objects which relate to the Question instance, e.g. q.choice_set.all()
If you don't like the foo_set naming which Django chooses automatically, or if you have more than one foreign key to the same model and need to distinguish them, you can choose your own overriding name using the related_name argument to ForeignKey.