Gamma is the linearity (or more specifically lack there of) of the effective brightness of the display output versus electrical input drive level.
It can be compensated for by providing an inverse of the 'bow' to effectively create a straight line linear response. Just about all displays, whether they are LCD based, CRT based, OLED based, or Plasma based, need some gamma correction to provide the correct brightness/contrast response.
Red, blue, and green display components have different gamma corrections.