request for explaining code on form's reference

Question

I have a main form called Form1 and a child for called Form2.
I put the code below in Form2's declaration as a reference to Form1. 

private Form1 m_parent;

public Form2(Form1 frm1)
{
InitializeComponent();
m_parent = frm1;
}

My problem is I cant see any reference how Form1 can be tied to Form2.
As far as I know m_parent and frm1 are a type of Form1 in this code
but can this refer to Form1?

Any explanation would be appreciated.

Tamas