What do the four rings mean in the Audi logo?

Answer 1

The Four Ring Symbol
The Audi symbol is four overlapping rings that represents the Auto Union. The Audi emblem symbolizes Audi amalgamation with DKW, Horch, Wanderer, and NSU : the first ring represents Audi, the second represents DKW, third is Horch, and the fourth and last ring Wanderer.

Answer 2

Audi adopted this logo after the merger of four companies: Audi, DKW, Horch and Wanderer in the 1930's. The combined company was orginally called Auto Union to reflect the merger.