The windowpane test is a bread-making technique that helps determine if a dough’s gluten network is properly developed. In other words, if you pass the windowpane test, then the dough is a strong dough, can hold its shape and the dough’s gluten network is properly developed.
After mixing and kneading, you stretch out the dough as thin as possible (without tearing it) and see if you can see light passing through.
Similar to how sunlight passes through a windowpane.