The quick answer is that blue light is short wavelength and high energy light that can damage cells in your eyes. I like to compare it to UV light that can be emitted from commonly used devices such as iPads, phones, tablets, and computer screens.
The main source of blue light exposure is the sun and it is for this reason that we highly recommend UV protection for your eyes to prevent macular degeneration. Other sources of blue light are fluorescent lights and LED lights. As I mentioned above, the devices we use for work, play, and school are commonly backlit with LED lights and expose us to blue light.
This excess exposure is known to affect our sleep cycle. That is because in high blue light conditions (daytime) melatonin secretion is suppressed making us more awake and alert. Conversely, in low blue light conditions (night time) melatonin secretion is increased and we become more drowsy and ready to sleep. As you can see, by exposing ourselves to the LED light at night, we are throwing this natural cycle off which increases fatigue, decreases our ability to focus, and can lead to depression.
Limiting screen time, taking breaks, and considering blue light filtering lenses are the best ways to reduce the negative effects of blue light exposure.