What is the meaning of a reported OD600 value?
For most laboratory spectrophotometers the path length in cuvette that holds your sample is 1 cm, and unless another path length for the light is specified you can always assume that the Beer-Lambert Law for absorption of light is using 1  cm.

When growing a bacterial liquid culture in an incubator, the OD (wavelength is usually 550 nm or 600 nm) is a approximation of the number of cells, or colony forming units, or cfu, per mL of culture.

The recipe, or protocol, will normally specify the bacterial strain to use, and the desired OD (at which point one either harvests the growing cells, or possibly adds something else to the culture, like a drug, or a viral stock).

To actually know the number of cfu/mL for a specific bacterial strain one needs to construct a growth curve by taking aliquots at defined time points, and reading the OD, and also plating out some serial dilutions.  After incubating the plates at the correct temperature one can count the number of cells per mL and plot cfu/mL vs. OD600 (for example).