Help with fields_dict[]

Can anyone tell me what does fields_dict[] do?
e.g what does it stands for in

$(frm.fields_dict['fieldname'].wrapper).html("This is ERPNext discussion forum")

It holds the control objects of each of the form fields.

A field’s control object is pretty core, allowing you to control everything from the field’s value to it’s bare jQuery element.

However, the intent of the usage you state isn’t very clear. It basically sets the entire wrapper element of the field to a different value. Could you let us know the contextual use case?

Actually I just want to insert a template into a Form.
I want to do as described in this post by Revant One: