Optical Density numbers If it's OD of 600nm I know that's the wavelength used, but what are the other numbers in something like  "OD 600nm of 0.6-0.7". The 0.6-0.7?
That is the optical density (absorbance). It's just a range of between 0.6 and 0.7. Without the context, it's hard to say why they don't have a precise value, but in something like a protocol, it's common to see that kind of thing when there is some natural variation or the exact number isn't important.