This article is from the Hiking Boots FAQ, by Chris Gilbert firstname.lastname@example.org with numerous contributions by others.
A midsole is piece of stiffened material, usually nylon, that is
incorprated in the sole of the boot. If you intend doing a lot of
hill walking then a boot with stiffened midsole will help
considerably in stopping your feet from getting tired too quickly.
Some boots have significantly stiffened midsoles and these will also
permit the use of walking crampons if you are going to go winter
walking. If all you are going to do is low level walking then a
moderately stiffened midsole will probably prove sufficient when
something quite stiff will probably prove uncomfortable.