Zero Drop refers to the height differential between the shoe heel and toe area. Most conventional running shoes have an average drop of 12 mm, which places too much impact and stress on the heel region-- and we know what happens next and it's not good for runners. The runner's cushioned heel strikes the ground first instead of the shoe landing on the more biomechanically efficient midfoot or forefoot. Excessive heel-striking marks an open-invitation for potential foot and leg injuries. But minimalist and barefoot running shoes have a much less drop. Which means you are now landing on the midfoot or forefoot-- the way nature originally intended us to run. Approaching zero drop with your footwear will get you as close as possible to natural or barefoot running. Zero drop has become the ultimate goal for running shoe companies and runners alike.
The popular Vibram's FiveFingers is a zero-drop barefoot running shoe. Saucony has a new zero-drop shoe called the Hattori. Footwear upstart Altra, who originally coined the term, has several zero-drop models for the trails and roads this Spring. But the majority of runners, who are used to running in shoes with huge built-up heels, aren't ready to suddenly go all the way to zero drop (which can also cause foot and leg injury if done too hastily because the joints, tendons, and muscles need sufficient time and training to make the new adjustment); hence footwear companies are busy developing intermediate or transitional shoes of varying heel-to-toebox height differences. These transition-style shoes like the popular Nike Frees are ideal for runners looking to safely, and without injury, gradually make the change to a healthier and more natural style of running.
To paraphrase Bob Dylan, "The shoes they are a-changin'". Or to paraphrase the late Timothy Leary, "Tune in, turn on, zero drop."