A data group is a subform (a form that we inserted in another form).
The primary form is the main form, and the form that we enclose in the form is the subform. We sometimes refer to a form/subform combination as a master/detail form or a parent/child form.
Subforms are especially effective when you want to add multiple records to a table.
For example, you can create a form that displays customer and order (date, ship date, etc.) data and contains a subform that displays the products (product, unit price, etc.) of the customer’s order.
The article “Create a subform (editing subform)” explains how to create one.