The number is power output in watts/bodyweight in kilos.
To determine your power output you need to time yourself doing a certain measured effort. A flat out performance on the flat, or riding a hill that you know the length and % incline of.
Then put it into this calculator
The important thing to take into account is the time categories, because you can put out much more power over 5s than FT (continual effort).
For me, I only have figures for last year, where I put out 270 watts for just over an hour up Alpe d'Huez. My weight was 67 kilos, so that's a P/W ratio of 4.02 FT power output, putting me (last year) between 'good' (cat 3) and 'very good' (cat 2). Armstrong did Alpe d'Huez putting out 516w in the time trial. He was 75 kilos, giving him a P/W ratio of 6.88 - i.e. off the scale.
Where do you sit?