All light is made up of different colors, digital devices tend to have a higher proportion of high energy visible (HEV) blue light. The higher the energy associated with the light the more it has the ability to affect us. Blue light and Ultra Violet (UV) light are on the high energy side of the spectrum and are able to cause changes such as a suntan or damage to our eyes.
The simple answer is that blue light is a natural part of almost all light, whether it is light from the sun or light from the bulbs in your house and office. What makes digital devices a serious concern for eye doctors worldwide is the proximity and total time that the average person is exposed.
Our eyes are particularly bad at filtering out high energy blue light from entering and affecting the more sensitive areas of our eye such as the retina. Eye doctors and researchers agree that blue light in the sensitive retina causes damage, however there is little research done on the long term effects of years spending hours per day in front of a cell phone or computer.