Drop
Drop is the difference in sole height between the heel and toe of a running shoe, measured in millimeters. Classic running shoes have a drop of 10-12 mm, while 'zero drop' (0 mm) mimics a barefoot foot position.
In detail
A higher drop shifts some of the load from the Achilles tendon and calves to the knee, while a low/zero drop, conversely, requires more active work from the foot and lower leg. An abrupt change from a habitual drop (especially switching to a low drop) is a common cause of Achilles tendon and plantar fascia overload in unprepared runners.
How to use it
Change your running shoe drop gradually, over several weeks, introducing new footwear into rotation little by little — an abrupt switch from 10 mm to 0 mm without adaptation is a typical cause of injury.