Annihilation's secondary focus comes down to not what the Shimmer is but what the Shimmer does. Determining this is the primary goal of Lena's team, made up of scientists working in various fields. Until Kane, nobody who entered the Shimmer ever returned (and he himself seems to be sick, no longer fully in control of himself — clearly in pain, under some sort of control of the Shimmer), so the Area X crew can't say for sure what happens in the Shimmer. As Lena and her crew journey in, they begin to realize what it is the Shimmer does.
The Shimmer isn't an explicitly destructive force, as it at first seems to be. The land inside is no ravaged wasteland but rather a lush, exotic forest filled with surreal flora and fauna and animals that only vaguely resemble those we see on Earth. The scientists discover that everything exposed to the Shimmer is in a constant, accelerated state of mutation. Everything is constantly evolving, growing, and changing. What's more, that change and mutation isn't confined to singular DNA sets.
Scientist Josie (played by Tessa Thompson) is the one that realizes that the Shimmer refracts DNA the way a prism refracts light. This means that mutations amongst DNA spread throughout the environment, creating crossbreeds between humans, animals, and plants — a sort of "species blend," such as the alligator with shark teeth or the bear crying like a human when it means to roar. By refracting DNA, the Shimmer creates a world in which everything is interconnected on a biologic, genetic level.