The dude who invented it says that your blood reacts differently to various types of food depending on your blood type, with some foods causing negative reactions. I’m type 0 so got to eat lots of meat. Worked for me 
Here’s a high level copy past from web MD:
O blood type diet
This is a high-protein diet that’s heavy on lean meat, poultry, fish, fruits, and vegetables, while light on grains, beans, and dairy. If your goal is to lose weight, you’ll want to avoid carbohydrates. Instead, choose seafood, kelp, red meat, broccoli, spinach, and olive oil. D’Adamo also suggests taking various supplements to help with digestive troubles and other issues he says people with type O blood tend to have.
A blood type diet
Type A blood types are advised to stick to a vegetarian diet full of fruits and vegetables, tofu, beans and legumes, and whole grains — ideally, organic and fresh — because D’Adamo says people with type A blood have a sensitive immune system.
Vegetables, pineapple, olive oil, and soy are best for weight loss, while dairy, wheat, corn, and kidney beans may lead to weight gain, he adds.
B blood type diet
If you have type B blood, a mixed diet that includes meat, fruit, dairy, seafood, and grains is thought to be best. Foods to avoid include corn, wheat, buckwheat, lentils, tomatoes, peanuts, and sesame seeds. Chicken can also be problematic, D’Adamo says. The list of foods to eat more of includes green vegetables, eggs, certain meats, and low-fat dairy.
AB blood type diet
Foods to focus on for this blood type group include tofu, seafood, dairy, and green vegetables. D’Adamo believes that people with type AB blood tend to have low stomach acid and because of that, should avoid caffeine, alcohol, and smoked or cured meats. Some dairy products, such as yogurt and kefir, are also good for this group.