Think of looking at objects at the bottom of a swimming pool while the water is stirred; the ripples in the water make the objects appear to dance or shimmer, similar to how air currents make stars twinkle.

STARS: S-tars T-winkle A-tmospheric R-efraction S-hift

The Earth's atmosphere consists of layers with varying temperatures and densities. According to Snell's law, $n_{1} \sin \theta_{1} = n_{2} \sin \theta_{2}$, light bends when passing between media of different refractive indices. As starlight passes through these fluctuating air pockets, its path deviates continuously, causing the star to flicker to an observer on the ground.

Remember that planets are much closer to Earth than stars and appear as extended sources, so the light variations from different parts of the disc cancel out, preventing twinkling.