Gamma is a measurement of how the luma range turns into brightness when it is displayed. Although black and white will look the same under different gamma values, the middle of the range will look different. Different computer platforms and TVs all have different default gammas (though Windows is very close to TV). Apple computers use a value of 1.8 by default, whereas Windows uses a range from 2.2 to 2.5 (TV is 2.2).
As a result, some video may appear to have brighter midtones on an Apple computer than on one running Windows. If you are encoding for a mixed environment or are unsure of what playback device the end user has, it's best to split the difference by targeting a value of 2.0.
Brightness and Contrast
Brightness filters adjust the overall intensity of each pixel in a frame by a fixed amount. Contrast, on the other hand, increases or decreases each pixel's value by an amount proportional to how far away from the middle value it is (the farther away, the bigger the change). These values are often used together, rather than separately, because just brightening an image without adjusting the contrast can leave the black levels too muddy gray, rather than the desired black.