The aurora borealis is a phenomena visible in the far north of our world and Lyra's.
An aurora is a natural light display, best seen in the polar regions, caused by the collision of charged particles directed by Earth's magnetic field.
In the Northern latitudes this is called aurora borealis after the Roman goddess of the dawn, Aurora, and the Greek name for the North wind, Boreas. It is also known as the Northern Lights.
The aurora becomes central to the plot of the novel, hence the British title of the novel.