ShowMessageDialog - Components

I thing my question is very simple for you. I am recently starting with programming. I woudl like to know exactly what it's doing the null of the showMesageDialog(null,"string").

I always see that people set null, but I don't know exactly why is the reason to put null.

I put an example.

Hope you can help me.

"JOptionPane.showMessageDialog(null, "ERROR DE PENSION");"

//1. Definicion de variables

    int pension;
    String categoria;
    Double Promedio;
    Double Descuento;

    categoria = txtTP.getText().toString();
    System.out.println("hola" +categoria);
    if (categoria.equalsIgnoreCase("A"))
        pension = 550;

    else
        if(categoria.equalsIgnoreCase("B"))
            pension = 500;

        else
            if(categoria.equalsIgnoreCase("C"))
                pension = 460;
            else
                if (categoria.equalsIgnoreCase("D"))
                pension = 400;
                else
                pension = 0;
                JOptionPane.showMessageDialog(null, "ERROR DE PENSION");