Heel-to-toe drop is the difference in sole height between the heel and the ball of the foot. Conventional sneakers and running shoes often have a drop of 10 to 15 mm.
A raised heel changes the position of the foot. Moving to a different sole design is a change that should be approached gradually and according to your individual needs.
Zero-drop means a difference of 0 mm: the heel and forefoot sit at the same level.