Whenever light hits particles, either in our atmosphere or in space, some of that light is scattered and some of it gets reflected back relatively strongly in the direction it came from.
Think of shining a torch in a dusty room. Those either side of you will see the light being scattered off the dust, but you will see the dust much more brightly, as it is being reflected straight back at you, the torch holder.
The ‘Gegenschein’ is the name of this effect when the sun’s light bounces of dust in the solar system, back in the direction of the sun. If the sun is behind us, which is what we mean by nighttime when looking at the heavens (the sun is behind us and the Earth is between us and the sun, hence it’s dark), then we can catch this glow of distant dust in the night sky.
The Gegenschein glow will always be close to the antisolar point, which is where this gets interesting for natural navigators.
Since we can make a good estimate of the bearing of the sun, working out where the antisolar point is is very straightforward, it is opposite, ie. 180 degrees from that bearing.
So we can in theory use the Gegenschein to navigate.
A simple example:
The sun will be close to due north at midnight, therefore if we pick up the Gegenschein, we are likely to see it due south.
Of course, the other way of looking at this area is this: if you already have your bearings, you can look in the direction of the antisolar point to improve your chances of spotting the Gegenschein.
As so often is the case, we can either use a natural phenomenon to help us find direction or direction to help us find a natural phenomenon.