system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the difference between brightness, contrast, and gamma?
gpt
These are terms relating to video, both analogue and digital.  They are settings that affect how a TV or other display device shows the video information it's receiving.  Let's say the video has the raw brightness in the range from 0 to 100...

The TV's brightness setting controls which level is considered black.  Usually the correct setting is around 6 and anything from that level or lower will show as black.  Turning the TV brightness setting up higher sets the "black level" lower which will make the picture brighter on the screen but can also reveal dark information not intended to be seen, like the sync information on analogue video.

The TV's contrast setting controls which level is considered white and is usually around 92.  Turning up the contrast sets the "white level" lower, exaggerating the difference between the brightest and darkest part of the screen but also hiding detail in the brightest parts of the image.

A TV's gamma affects how the video levels correspond to the brightness of the light coming from the screen.  You might think that video level of 10 would show 10 times more light than a level of 1 but instead, the amount light goes up according to the video level taken to the power of gamma.  If the gamma is 2, the video level 10 shows 10^(2) as much light as video level 1.  The NTSC standard uses a gamma of 2.2 and PAL has 2.8.

The reason for gamma is that the human eye can see a very wide range of brightnesses and using a power or even an exponential function to represent levels better corresponds to the way the eye works.  It gives a better quality image for a given quality of video signal.