It is the id of the inline element on HTML page. You can check the id of the default HTML inline element.
<div id="[related_name of ForeignKey]-group">
For example: If in model "MyModel2", you have a ForeignKey like this:
my_model_1 = models.ForeignKey(MyModel1, related_name='my_model_2')
Then the id should be "my_model_2-group".