Why does the light from the stars twinkle because it is refracted as it passes though the atmosphere

Why does the light from the stars twinkle because it is refracted as it passes though the atmosphere. explain what this means and why it happens.
help please !

Yes. The atmosphere is roughly 200 km thick and due to temprature and wind in constant motion. So, the density is not uniform. The masses (lumps) of different density air move around. Different density means the different masses have different Refractive Indexes, so the light gets bent different amounts as the density changes. This makes the stars appear to twinkle