It refers to the number of shots you are able to take before your buffer runs out.
So say, when you half-press, you see [r09]. That means that you can continuously take 9 shots before your buffer is full, and you'd have to wait for your buffer to clear before you can take another shot. So when you shoot continuously, you should see the numbers decreasing from [r09], [r08], [r07]... but since the buffer clears as you shoot, there may be a case where the number goes like r09], [r08], [r07], [r07], [r06], [r05], [r05]...
The quality of your image determines how many pictures you can take before your buffer is full. Shooting in TIFF or RAW mode will typically get you less than shooting in Small Jpeg.
Or refer to your user's guide. Page 9 number 17.