In this lab, you can explore what happens to a wave traveling through one medium. If the wave encounters a fixed end, the wave is reflected upside down. If the wave encounters a free end, the wave is reflected upright.
If the wave is on a light string that is connected to a heavy string, the interface acts as a fixed end, and the part of the wave that reflects is reflected upside down.
If the wave is on a heavy string that is connected to a light string, the interface acts as a free end, and the part of the wave that reflects is reflected upright.
The transmitted part of the wave is always transmitted upright.

This is relevant for light when it encounters an interface between two boundaries. The light string is analogous to a low-n (fast) medium, and the heavy string is analogous to a high-n (slow) medium.