If you don't knead your bread dough enough, your bread will lack an airy, light texture. Over-kneaded dough gives you bread that breaks teeth. You don't want either, so the windowpane test is one good indicator for whether your dough is ready to become warm, delicious bread.
Image by arnold | inuyaki.
Lop off a golf ball-sized piece of dough. Flatten the dough, hold it between both your thumbs and first two fingers and gently spread the dough apart. If properly kneaded, the dough should stretch, without tearing or breaking, into a thin membrane that you can see through. The Kitchn also has pictures on what this should look like. Congratulations, your dough passed the windowpane test!
If not, then you probably need to knead it some more. This also works with pizza dough. Note that this test is most effective with dough that's been made with high-protein and finer flour. Whole wheat flour and other more gritty flours tend not to pass the windowpane test.
What is the Windowpane Test? [Make Bread]