If you’re not Superman you can’t see inside the human body. Therefore, humans, such as your doctor, have to use clues to figure out what is going on inside. That’s why you doctor thumped and listened. She was listening for clues with her stethoscope.
She wanted to listen to your heart — how often it beat and what kind of beating pattern she was hearing. She also wanted to listen to your breathing. And because you have two lungs, I’m sure that she listened to each side of your chest. Did they sound the same or different? When she thumped on your back, your doctor wanted to see whether the sound she heard inside was a hollow sound or more like a dull thud. A hollow sound would mean that there was nothing but air inside your lungs. That’s good. If she heard that duller sound I mentioned, you might have fluid or mucus inside your lungs — that would mean you had a infection — and that wouldn’t be as good.
As your doc listened to your breathing, she also wanted to see if it sound clear or if there was an accompanying rattle or wheezing sound. If she heard the latter, that might also have meant you had an infection or that your breathing tubes were constricted and you had a touch of asthma. I gather that you were fortunate. Your doc only found good hollow sounds, and breathing that sounded like the wind. That’s great because that sounds healthy to me!