Hi, here is my best explanation of this concept!
In lab, this is a common experiment that helps to illustrate the concept of oxidising and reducing agents. This reaction is simple to conduct and it is very obvious once the reaction has occurred via a color-change indication.
In this situation, the Potassium permanganate will serve as an oxidizing agent given its chemical structure while the Sulphur dioxide acts as the reducing agent to the KMnO4.
Given this, when these two chemicals react in solution, the result is a cancellation effect visibly illustrated by a color change from purple (KMnO4) to colorless (once the SO2 has been added).
I hope this helps!