The higher the concentration of bacterial in a liquid culture, the higher the optical density of that culture when measured. Organic material has high optical density at light with wavelength 600 nanometres, and light at this wavelength is in the middle of the visible wavelength and easy to produce as measure. The optical density of a sample at 600 nanometres is referred to as OD600. The measured OD600 of the sample relative to a plain reference sample, gives an estimate of the concentration of the cells in the liquid, which can be used to estimate the growth phase of the population.
The basic idea is to compare a sample of plain media, and a sample of media in which bacteria has been growing. Initially the levels of bacteria will be very low, and the amount of light absorbed by the 2 samples will be similar.