Well-kneaded bread should be like elastic. This is to capture the gases created by the yeast. The dough will stretch as bubbles form, then it will expand and rise. If the dough is not like elastic, then it will not provide chewiness or an open texture.
Two important proteins are contained in the endosperm of wheat - gliadin and glutenin. When the weat flour is combined with water, these proteins join with the water molecules and crosslink with each other as the bread is kneaded.
It takes a lot of kneading to make these molecules have contact and create strong links. As the bread is kneaded and the molecules create stronger bonds, this causes gluten to be formed. Gluten is the cause of the dough's elasticity.
If you use a stationary mixer, you can see what changes take place as the dough is kneaded. At first, the dough will stick to the sides of the bowl. A drier ball will be formed as the bonds become stronger and the dough gains elasticity. The sides of the bowl will now be clean. If the dough is mixed at medium speed, then within 4-5 minutes you will see the dough change even more as the gluten is formed.
If you pinch a section of the kneaded dough and pull, it should stretch into a thin layer before it breaks.