This is a custom-made insert which is worn inside a shoe to offer cushioning and support to the foot and/or to correct the way a person walks by spreading the pressure evenly around the foot making sure it hits the ground at the right angle. Sometimes insoles will be fitted just at the heel, but they can run the whole length of the foot. Generally, the maximum height they can be is 12-15mm. If there is a greater leg length discrepancy a shoe raise will be required.
This may seem like a modest device but an orthotic insole takes skill to design and create and, for cost of a carry out meal for two, it can limit or prevent long term damage to the shins, knees and spine.