In the neocortex, input patterns are compressed hierarchically. Sensory inputs in the lower levels are combined by higher levels to form abstract concepts. However, there are even more feedback connections ranging from higher levels downward to lower ones. Those are assumed to unroll higher concepts back into lower patterns that could have caused them in the first place.
If patterns could flow up and down through the same synapses, it would be easy to understand how higher patterns cause lower patterns similar to those that could have caused them. In a mathematical sense, traversing the connections in the inverse direction would cause the inverse of the upward transformation.
But synapses are mainly directed. So patterns cannot flow downward through the same synapses. How do the feedback connections still learn transformations similar to the inverse of the upward connections?