Many insects navigate by using light. In nature, almost all light is polarised, i.e. the rays of light run parallel to each other because the light source is so far away. This is true of sunlight and also Moonlight, which is reflected light from the Sun.
An insect's eye is a compound eye, made up of many small "mini eyes" called ommatidia. Each ommatidium points in a slightly different direction like a tiny telescope.
In order for an insect to fly in a particular direction, all an insect has to do is keep the light shining in one particular ommatidium and it will end up flying in a straight line.
The problem starts with light sources that are not polarised. Most man-made light sources, such as light bulbs (and fires) produce radiating light that shines out in all directions.
So when an insect tries to navigate using a radiating light source it has to fly in a circle in order to keep the light shining in the same ommatidium. The result is that the insect ends up circling the light in ever-decreasing circles.