Retrograde, reflection and retrograde reflection are techniques by which polyphonic music promotes mirror structures.

To put it simply:

The imitation of flipping up and down the sounds of the main sentence at equal distances (the interval relationship must be the same) is called reflection.

For example, sentence a is fa so la, sentence b is mi re do

And flipping the sounds of the main sentence back and forth at equal distances is called retrograde. This is easy to understand

Retrograde reflection is a combination of the above two.

You can ask me if you don’t understand.