These sounds are produced by the activity of different parts of the heart and different valves. However, contrary to popular belief, these are not exactly the sounds of the valves working.
The sounds occur when a valve closes and blood under pressure “hits” the valve. It’s like suddenly shutting off a pressurized hose—only much faster.
The first sound occurs when the mitral valve closes. It is quieter because the blood is in the atrium, where the pressure is lower.
The second thump occurs when the aortic valve closes. Here, the pressure is generated by the left ventricle, the “strongest” part of the heart. Accordingly, the thump will be louder.