By default, IntelliJ IDEA automatically creates a Java class at the same time it creates a new GUI form. The new form automatically binds to the new class via the bind to class property. When components are added to the design form, a field for each component is automatically inserted into the source file of the Form's class (with some exceptions such as JPanels and JLabels which don't automatically get field names when placed on a Form). The field name appears in the relevant component's field name property, thereby binding the component to code in the class. It only remains to augment the generated code with whatever additional code is needed to implement behaviors and functionality of the form.
Automatic binding of a component to a field is stipulated by the Create binding field automatically property of a component in the Palette.
You can opt to create a new form without a bound class. In this case you will have to explicitly bind a form to the source code.