In cooking, leaveners are agents that cause a dough, batter or other mixture to rise and expand by creating gas bubbles in it. This affects the texture of the baked good, making it lighter.
Leaveners are generally classed as “natural leaveners” or “chemical leaveners” (not that the chemicals involved aren’t natural ones, anyway.)
For example, yeast is a natural leavener; baking powder is a chemical leavener.
Unleavened baked goods don’t have the rising happening in them, and therefore are denser and heavier.