Insoles are a surprisingly important part of running-shoe anatomy. The insole is a lightweight, foam footbed that rests inside the bottom of the shoe to offer extra cushion and comfort for the foot.
All running shoes come with stock insoles—you can see yours by peeling them out of your shoes—that typically add another millimeter or two of cushioning.
You can buy running insoles separately for your shoes to give you different attributes than the stock insole. Some shoe inserts are rigid and supportive, and they’re shaped to influence your foot by providing arch support or other structure. Other insoles give you more cushioning in high-impact areas, like your heel and forefoot.