Bronchitis is an inflammation of the bronchi. This inflammation means the walls of your bronchi are swollen and filled with extra sticky mucus. Airflow into and out of your lungs is partly blocked because of the swelling and extra mucus in your bronchi. So breathing will become much harder anbd difficult. You may also cough up extra mucus.
With chronic bronchitis, the mucus cannot be cleared. Instead of helping to clean the lungs, the mucus blocks your airways. The mucus is thicker and more difficult to cough up. Hence it is easier for bacteria to settle in your lower airways and become infected.