GUI busy message

Question

I have a gui. When I press "button1" my gui starts serial comm. I send some commands and receive some data. Once I press "button1" I am in the other thread (other than GUI thread) for 3,4 sec untill data is fully transferred. What I want is a dialog box, pop up or some thing like that (showing a busy message) to appear infront of my gui when I press the button1 and this pop closes automatically in the other thread in which serial communication is taking place when the whole data is transferred.

Some people suggested back ground worker but I was not clear.

An good explanation or a way to do it considering my scenario ?

Thanks :)