From the infopedia:
When navigating star systems, you’re always occupying a point in space which is given coordinates.
These are an X and a Y coordinate where 0,0 is the very centre of the star system you’re in. Usually this is the star, but in binary star systems it can be a point somewhere between the two stars.
On the X axis, coordinates go up to the right, on the Y axis they go up as you go up. So coordinates of 50,50 would be 50 Gms to the right and 50 Gms up from the centre, while coordinates of -50,-50 would be 50Gms down and to the left.
The outskirts of a star system are generally about 500 Gms out from the centre of each star system. After the nebulae, planetoids and asteroid belts stop appearing, you’re in deep space and there is nothing out there, so it’s time to turn back.