Shoe drop refers to the difference in elevation between the toe and the heel. A zero drop shoe has no difference, the entire foot is the same distance from the ground – in other words it’s completely flat. That means that if a zero drop shoe is 10mm thick, it’s 10mm both under the ball of the foot and under the heel. Other terms used to describe the difference in thickness between the heel and the toe are “heel rise” and “heel to toe differential.”
You can find out why this matters in the How Do Barefoot Shoes Affect My Alignment FAQ.