Buck's dream is significant because it foreshadows his "retrogression." It's a dream which recalls the ancestory of his breed and "calls" him back to his more primitive roots.
With regard to "retrogression," in order for Buck to move forward in his "development," he must actually, go back toward a more primitive state. In other words, to move forward, he must go "backward." Sort of an interesting concept, but one that makes sense once you realize London's attempt to call into question the whole idea of what it is that we call our nature.
In Buck's dream, he recalls a time when his ancestors lived amongst primitive man. Prior to arriving in the Yukon, Buck lived a more "civilized" existence. It's this dream which reminds Buck of his true nature and acts as an impetus for his liberation.
The first time Buck has the dream, it's not clear to him what its significance is. As the dream occurs subsequently, he's able to make more sense of it and can move more toward the realization of his maturation.