What does "gamma" mean in photo editing?

Gamma is an important but seldom understood characteristic of virtually all digital imaging systems. It defines the relationship between a pixel's numerical value and its actual luminance. Without gamma, shades captured by digital cameras wouldn't appear as they did to our eyes (on a standard monitor). It's also referred to as gamma correction, gamma encoding or gamma compression, but these all refer to a similar concept. Understanding how gamma works can improve one's exposure technique, in addition to helping one make the most of image editing.

Adobe Photoshop is the industry standard, but also very expensive. If you buy a decent scanner or digital camera you will get a free copy of Adobe Photoshop Elements. (You can also buy this for about 50$-100$.) I would recommend Photoshop Elements because it is more user friendly, easier learning curve and all around a better price for just starting out.

Gamma is about contrast. In film photography, if you wanted to increase gamma, you could over-develop. Likewise, high gamma film would be high contrast. Why they'd put that in a phone app is beyond me. Gamma is a very technical way to measure contrast (the ratios between highlights and shadows) as it relates to exposure. It seems to me that a simple hi-lo contrast slider would do just fine. But if you want to play with it, make small adjustments. A slightly higher gamma might give a pic a little more "snap", say if you shot on an overcast day. It seems to me that there'd have to be an exposure adjustment as well. You might need to back that down a bit if you increase gamma. If a pic is already too contrasty, usually there's no saving it. The information is already lost.
OTOH, could be that "gamma" is just the fancy name they gave their contrast adjustment.

Gamma is a type of light (I think) and gamma adjustment probably is used for that.
Probably better to ask this in the computer and electronics forum.