Gamma correction is a method to prevent the monitor from displaying images that are too light or too dark. Monitors, due to basic physics in the way that they display colors, have a curve that defines the input value of a color's brightness as compared to the actual brightness of the color displayed. Because of this curve, low brightness colors have a tendency to appear too dark, whereas bright colors have a tendency to appear too bright. To fix this, a small calculation is made before the brightness value is sent to the monitor to make this curve as much of a straight line as possible. This way a designer can make a color as bright or a dark as he or she wants, and it will be displayed the same way on every monitor, if that monitor has been gamma corrected.
Most video cards have a slide that can adjust the light levels in a game or application.
However, Apple has gamma correction built into the Mac OS, so you may not need this feature. Also, many games themselves have brightness controls built in, so gamma correction is really an issue for older graphics cards and monitors. There are a number of ways to smooth these jaggies out, and these techniques are called anti-aliasing. Jaggies are a result of using pixels. Unfortunately, as long as we use little colored squares to draw images, we will have pixelation.