It's a way to determine gluten development/formation in your dough.
Gluten development is tested with the “windowpane test.” Pinch off about two tablespoons of dough and try to stretch it into a thin membrane (windowpane).
If you can do so without tearing, but the membrane is mostly opaque, you have barely developed gluten.
If you can stretch a paper-thin, very translucent windowpane, the gluten is fully developed.
A medium level is in between these two extremes: the windowpane is translucent with some opaque areas.