When people make bread, they often add yeast to the dough. Yeast is a very small living thing. It works well in warm, wet dough.
Yeast uses sugar in the dough. As it works, it makes a gas called carbon dioxide. This gas forms small bubbles inside the dough.
As the bubbles grow, the dough rises. Later, heat in the oven makes the dough firm. The bread keeps its risen shape, so it becomes light and soft.
Without these bubbles, bread would be heavier and harder.
For this reason, yeast plays an important role in making bread feel soft and airy.