Masking

Hi

I want to draw a line from the center position of a circle (transparent filled, just a stroke ring) to the center position of another circle (also transparent filled, just with a stroke ring). But I want to show the line connecting one ring with the other... so I need to mask with a circle the transparent fills of the circle rings... I've been trying lots of things but noone works for me... Could you explain the mask functionality please!? Thaaanks!!!