IBUs divided by Original Gravity points
so if your IBUs were 35 and your original gravity was 1.048, your bitterness ratio would be 35/48 = 0.729
It gives you some idea of the balance of the beer. More malty and higher ABV beers (higher OG) generally need more IBUs to balance it out. Different styles of beer tend to have a range of bitterness ratios that are "standard" for the style - more malty styles (e.g. Irish Reds) have a low bitterness ratio whereas more bitter styles (e.g. IPAs) have a high bitterness ratio . I remember seeing a list somewhere that gives target bitterness ratios for different brews. I'll see if I can track it down.
[EDIT: see next response]