Computer monitors and mobile/tablet screens do their best to reproduce colors and brightness correctly, but each one is different. In fact, a screen even reproduces photos differently when it starts up compared to the way colors and brightnesses look after the monitor has been running for a while.
This is a serious problem when viewing photographs on the computer, mobile or tablet. Photographers are careful to adjust the photographs properly, so they print perfectly, but other users will not view these nuanced adjustments at all on their own screens, and in fact may view the photograph totally wrong (overly bright, too contrasty, or oversaturated/unsaturated). There is no way a user will see the image as the photograph will print, unless their own monitor has been professionally calibrated with a special calibrating device (this of course is very uncommon unless you are a professional photographer yourselves). Therefore, what you see on your screen is really not what you will see once you receive the prints for your photographs!
Posted in: Colour calibration