Drop is the difference in height between the heel of the shoe and the front part of the foot. So in other words, a shoe that has zero drop is flat. It is the very opposite of having a heel of any kind on one's shoe.
A shoe construction with zero drop creates a more natural design for how our feet act without shoes. It is therefore not surprising that most barefoot shoes have a very small "heel" or are usually zero drop. It's similar to simply being barefoot.
Better posture with flat shoes
A shoe that has zero drop allows your posture to be more upright. A shoe that has an elevation in the heel places a higher load on the front part of the foot.
Thin soles take time to get used to
It may take some time to get used to using a flatter shoe with zero drop. It's good to start gradually and let your feet wear bare shoes in stages, rather than drastically changing all your shoes.