The bread or idly dough rises because of the growth of yeast cells. This process is known s fermentation.
The process of fermentation
The yeast consumes the sugar present in the dough and burps out carbon dioxide gas and ethanol. This gas gets trapped inside the bread dough due to gluten in it, thus making the dough rise. The alcohol gets evaporated in the baking process. During bread making, the dough is left in a warm place. During fermentation, carbon dioxide is produced and trapped as tiny pockets of air within the dough. This causes it to rise. If you let the dough rise for too long, the finished bread’s taste and texture suffer.