When you have an asthma episode the muscles wrapped around the tubes inside your lungs squeeze tight. When the muscles squeeze, you can't get air into or out of the tubes, and you can't breathe. Then the tubes become inflamed and fill up with thick, sticky mucus which clogs them up. This can make it really hard to breathe - OUCH!
Try this experiment to see how hard it is to breathe during an asthma episode. Find a drinking straw and blow air through it. Now pinch or bend the straw and try to breathe through it. Air doesn't go through easily, does it? When you have an asthma episode your lungs cannot do their job.