Nightsiders are Abhumans who live out their lives in near or complete darkness on worlds that either are so far from their primary star that little or no light is shed or planets that do not rotate and so one side is always facing away from the sun. Nightsiders may have developed larger eyes than a normal human, almost bulbous, so they react badly to "normal" light levels from absorbing too much of it; others may have vestigial eyes with their other senses greatly enhanced to compensate for the near-blindness, so they react poorly to other "normal" stimuli. Or, of course, both - "nightsider" is a general term for a broad array of possible adaptations to low-light conditions, including developing more exotic senses like echolocation (like a dolphin), electroreception (sensing electricity, like a shark), magnetoreception (sensing magnetism, like a pigeon), etc. Their adaptation to the dark typically makes their skin extremely pale, as the lack of sunlight has lessened the amount of melanin in their skin, making them extremely vulnerable to sunburn should they be exposed to higher light levels.
They are most commonly found in Night Worlds where there is very little to no sunlight on the planet. Other than that, not much is known about them despite being very common on planets in the Imperium. Some have come up with the idea that Nightsiders may act as nocturnal infiltrators or assassins for the Imperial Guard while others may turn them into glorified electrician repairmen, sewer cleaners, underground exterminators, or any job that has a lot of nocturnal activities.
Back in 3rd edition, you could get your own army of night-vision Riddick warriors through GW's Chapter Approved rules. With the right army doctrine, for 10 points per squad, any Infantry unit of Imperial Guard could get Acute Senses. Unfortunately, this rule has not aged well, as Acute Senses only really applies when outflanking units nowadays, but back in the day it also impacted how Sentries worked when the battle missions meant raising the alarm whenever enemies were spotted.
Nowadays you would probably spend 0 points per squad to let them ignore Stealth and Shrouded, and require any Nightsiders unit to take a Blind test on any turn when *not* Night Fighting.