Baking powder is a mixture of baking soda and an acid (usually cream of tartar) that's designed to release gas more slowly over time. When it's fresh, this process happens smoothly, resulting in a beautiful, golden-brown crust. However, as baking powder expires, this reaction slows down, leading to a less-than-desirable texture.
A good way to think about it is like a firework - when it's fresh, the colors are vibrant, and the explosion is impressive. But when it's old, the colors are dull, and the explosion is more like a fizzle. You want your baked goods to be like the former, not the latter!
