Gluten is formed when naturally occurring proteins present in flour come together to form the stretchy networks that give baked goods their classic structure and texture. These protein networks can trap air, as in bread, or create a chewy texture like that in artisan pizza dough. Typically, the more protein a flour contains, the more gluten it can form. 

Kneading promotes the creation and strengthening of gluten strands, as does a higher moisture content. This is why no-knead breads have a relatively high hydration; the water does most of the heavy lifting in terms of gluten formation, and kneading isn’t necessary. This is also why we avoid overmixing tender products such as pancakes and quickbreads. The extra agitation would promote gluten formation, yielding a dense, heavy product.