Atmospheric Dispersion Corrector

The ADC is a device with two prisms that correct the prism like effect of the lower part of the atmosphere. All objects below 60 degrees get a distortion like chromatic aberration because at those low positions the path of the light is affected as red wavelengths reach further than the blue ones.
The ADC works best with scopes with an f10 or slower so it helps to place a Barlow before the ADC as it makes the focal length longer and therefor the f-value higher.

I won't even try to explain further as the next videos got that spot on:

How an ADC works and what the ADC does.

Clear instruction by Jenham's Astro.