As with everything else, there are two kinds of people. Those who have made home-made bread, and those who haven't. Those who have can tell you that it's pretty easy. Those who haven't, seem convinced that it's a task requiring skill.
Let's start with something around the level of High School biology. Yeast is a fungus. It likes to eat, and reproduce. In fact, it's a lot like most college students in this regard.
The presence of food and warmth are basically all you need to encourage yeast to engage in reproductive activity. Again, not much different from college students.
During periods of vigorous reproductive activity, a lot of carbon dioxide is generated. And with yeast, this is also true.
Flour and water are the basics of food for yeast, but sugar works very well, too.
This is a measuring cup that has 2 heaping teaspoons of brown sugar, dissolved in warm water, with around 2 1/4 teaspoons of yeast stirred in. Watch what happens within 10 minutes.
Note the generation of a huge pile of foam-->
(Evidence of reproductive activity... yours for the viewing on the internet. Again, not unlike some college students.)
Bread is a mixture of flour and water, with yeast inside. And the yeast will continue to happily reproduce within an environment that is almost entirely composed of food.
This is the magic of bread making: Think of yeast as a bunch of college students. Provide food, and adequate warmth, and they're bound to do what comes naturally. The rest is just the process of mixing, waiting, and turning up the heat.
Think of the oven as Spring Break in Mexico. It's hotter, there's a final reproductive blowout, and then everybody gets thoroughly baked.