So, the meaning of “zero-drop” is the angle between your heel and your toe when it’s in a shoe. That’s all. Most shoes, such as tennis shoes, heels, or work shoes (or boots) place the heel higher than your toes and there, hence, “drop” your foot down at an angle. With zero-drop shoes being worn places your heel and toes level with each other. This is the same as if you were walking around barefoot.
