//button on form1
button1_Click(object sender, System.EventArgs e)
{
string LabelText = "this will show up in the label";
Form2 f = new Form2(LabelText);
f.Show();
this.Hide();
}
==============
//Constructor on form 2
Form2(string LabelText)
{
InitializeComponent();
label1.Text = LabelText;
}