OD600 is an abbreviation 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.
Basically the wavelength of measurement (WM) of OD depends on growth of the culture. If you expect "huge" growth then you make WM longer that you may still stay in a linear part of relation between cell number and OD (and you do not need to dillute the culture to get a reliable value). If you expect low cell number then you need to be more sensitive in OD measurement then you might go to a shorter WM. The choice of WM you can also make on the basis of knowledge of pigments - for example, cytochromes etc. and of course, you need to consider if the medium you use has color and the medium color is changing during growth of microorganisms. If color of the medium is stable then you may just use the medium as "0" OD.