Imagine a jar of gooey flour-and-water paste with tiny invisible creatures living in it, too small to see. They eat the flour, and when they eat, they burp. The burps are little bubbles of air.
When you mix that paste into a big lump of dough, the dough is stretchy like chewed gum. So every burp gets stuck inside and pushes the dough bigger and bigger, like blowing up a balloon very slowly.
Then you put the puffy dough in a very hot oven. The heat makes the bubbles push one last time, and then the dough goes hard and stops moving, keeping all the holes exactly where they were. That's bread. If you wait too long before baking, the creatures eat so much that the stretchy dough gets weak and flops. If you don't wait long enough, there aren't enough bubbles and the bread comes out heavy.