Humans can only get fresh air in their lungs when they breathe in. Above is a simplified diagram of the bird respiratory system. The actual lung itself is the small, thin part between the two larger air sacs. As air passes through, oxygen is removed. In real life, air is continually passed through the lungs, but this diagram only shows one breath as it travels through the bird.
Bird lungs are much better at getting oxygen out of the air than human lungs.
This is important for birds, because they use up a lot of energy when they fly, so their cells need more oxygen to get the energy out of the food they eat. Also, birds sometimes fly very high up, where there is very little oxygen in the air. When humans climb Mount Everest, they need oxygen tanks to help them get enough oxygen.