Say Form1 and Form2 are two classes. then create an object of Form2 in Form1.
In the click event of button try this.
//Open the Form2
//Make the Label/TextBox where you want to display the listbox value visible
_Form2Object.LabelName.Text = ListBox1.SelectedValue
Hope this helps.
Try providing better explaination or code snippet, so that you can be served better.