You first need to decide under what circumstances you want this to happen. The most likely options would the MouseEnter and MouseHover events of the Label or whatever you happen to use. You then need to decide whether you want to display controls on the current form or display a new form. If the first option, you would likely call Show on some control, which may be a Panel containing multiple other controls. If the second option, you would likely call Show on a borderless form. There are other options too. You would also have to decide exactly when you want to hide the extra content. Probably on the appropriate MouseLeave event.
I'd suggest that you first determine EXACTLY how you want the whole thing to behave, then you can look at the specifics of implementation.