The right end of a string is tied down - this is what we call a fixed end. When a wave traveling along the string hits the fixed end, what happens?

1. It reflects and comes back in the same orientation
2. It reflects but comes back inverted
3. It is completely absorbed by whatever is holding down the fixed end

A wave on a string is flipped when it reflects from a fixed end, and is reflected upright (no flip) when it reflects from a free end. Flipping the wave is equivalent to shifting it by half a wavelength.

Now consider a wave traveling along a light string tied to a heavy string. At the interface part of the wave is transmitted and part is reflected - the interface acts like a fixed end, so the reflected wave is inverted.

For a wave traveling along a heavy string tied to a light string the reflected part of the wave is not inverted when the wave reflects.