OD600 is an acronym indicating the absorbance, or optical density, of a sample measured at a wavelength of 600 nm. Also seen written as "O.D. 600," "o.d. 600," "OD600" etc. It is a common method for estimating the concentration of bacterial or other cells in a liquid.
Concentration tells the stage of cultured cell population. The stage of population tells if the population is in lag phase, exponential phase, or stationary phase. Value of concentration depends on used culturing medium.
OD600 is preferable to UV spectroscopy when measuring the growth over time of a cell population because at this wavelength, the cells will not be killed as they would under too much UV light.