According to the Impossible Foods website:
Our burger is made from simple, all-natural ingredients such as wheat, coconut oil, and potatoes. What makes the Impossible Burger unlike all others is an ingredient called heme. Heme is a basic building block of life on Earth, including plants, but it's uniquely abundant in meat. We discovered that heme is what makes meat smell, sizzle, bleed, and taste gloriously meaty. Consider it the "magic ingredient" that makes our burger a carnivore's dream.
Because we use 0% cows, the Impossible Burger uses a fraction of the Earth's natural resources. Compared to cows, the Impossible Burger uses 95% less land, 74% less water, and creates 87% less greenhouse gas emissions. And it's 100% free of hormones, antibiotics, and artificial ingredients.
You can watch Quartz break down the science behind the meatless burger in the above video. Then watch carnivores and vegetarians alike give it a try in this video from BuzzFeed: