What is a Data Group
A data group is a subform (a form that is inserted in another form).
The primary form is called the main form, and the form that is enclosed in form is called the subform. A form/subform combination is sometimes referred to as a master/detail form, or a parent/child form.
Subforms are especially effective when you want to add multiple records in 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.