The yeast comes alive and starts eating the sugars, creating carbon dioxide and alcohol, essentially it's burping and sweating, which is what bread is. It's yeast burps and sweat.
La levadura se activa y empieza a comer azúcares creando dióxido de carbono y alcohol, en esencia eructo y sudor que hacen al pan, es el eructo y el sudor de la levadura.