Digital technology provides the ability to adjust images in-camera or on a computer after capture. One area in which this can prove useful is in correcting for shortcomings and quirks in optical performance.
When lenses are designed, optical physics dictates what is possible, along with the price point of the lens and the complexities of manufacturing. In any case, it is fundamentally impossible to make a perfect lens – so, based on its design, every lens will exhibit a greater or lesser degree of optical irregularities. Typically, these manifest themselves as vignetting, where the corners of an image are slightly darker than the centre as a result of light fall-off, and chromatic aberration or colour fringing along high-contrast edges, where the lens has been unable to focus different colours or wavelengths of light to precisely the same point.
However, every design of lens has its own characteristic quirks and flaws. By mapping the performance of each specific camera and lens at a variety of focal lengths, focus distances and apertures, it's possible to correct for these irregularities and bring the lens performance closer to perfection.
These corrections were first made available in Canon's Digital Photo Professional (DPP) software, but the increased processing power of cameras has made it possible to carry out corrections in-camera as the images are captured if you're shooting JPEGs or during RAW processing in-camera if you're shooting RAW. You simply switch each correction on in the camera's Lens aberration correction menu.