Dough is a thick mixture of flour and other ingredients that can be kneaded, baked, and eaten. Bread dough needs to rise before you bake it in a hot oven.
Many baked goods begin their lives as dough, including bread, rolls, and some cookies. Dough is stiff enough that you can shape it, pull pieces off of it, stretch it and knead it. Informally, dough can also mean "money." If your friend says, "I make so much dough waiting tables!" he probably means money, not cookie dough. The word comes from an Indo-European root that means "smear" or "knead."