A subform is a form within a form. It is generally used whenever you want to display data from multiple tables where there is a one-to-many relationship.
For example you would use a subform if you wanted to display an order with the order details. In this case, the order information would display on the "main" form and the order details would display on the subform.
The following is a list of topics that explain how to use Subforms in Access: