A "neutral pelvis" means that your pelvis is in a balanced position. It is neither tilted forwards nor backwards. It helps to keep your body aligned and prevents excess strain on your back. Full definition
Then, if you're like many people, you'll have to DROP YOUR LOWER RIBS in order to be in a «neutral ribs over neutral pelvis» position. (femfusionfitness.com)
Neutral pelvis means that the lines from your belly button to your pubic bone and from hip to hip are level — imagine balancing a glass of water on your pubic bone, and avoid over tucking the hips so that the butt lifts off the mat. (womenshealthandfitness.com.au)