Declare a variable, use it in Program, Form1, ...

Hi,
 
Can you tell me what is 'internal static' in 'Program'.
I tried to put 'internal static string str;' between 'static class Program' and '[STAThread]' but the variable str looks not works in 'private void button1_Click(object sender, EventArgs e)' of 'form1.cs'.
Thanks.