If you hold a chrome mask up to a light, you can vaguely see the light shining through the chrome, much like the light leaking through a solar eclipse filter or welding goggles.
The purpose of the metallic coating on a photomask, such as chrome, is to attenuate the transmission of light enough that photoresist on the other side will not be activated. We want the coating to be thick enough to attenuate most of the light, but we also want it as thin as possible to get the best dimensional control on the mask since the thinner the chrome, the less etching is required. So decades ago, when the industry standardized on chrome as the absorber of choice, it was determined that an optical density of 3.0 was sufficiently opaque at common exposure wavelengths in the UV spectrum.
Technically, optical density is the amount of attenuation -- or gradual intensity loss -- that occurs when light passes through an optical component. Mathematically, optical density is a logarithmic scale of how much light is transmitted through the absorber material: OD = Log (Power transmission factor).
For example, an optical density of 3 attenuates the light power by a factor of 10^3 (1,000). An optical density of 4 attenuates the light power by a factor of 10^4 (10,000). This optical attenuation may result from not only absorption of light but also from scattering of light. Absorbance, a closely related term, considers only absorption within the optical component but not scattering.
Transmitted Light Optical Density
100.00% = 1.0000 = 1x10^0 0
10.00% = 0.1000 = 1x10^-1 1
1.00% = 0.0100 = 1x10^-2 2
0.10% = 0.0010 = 1x10^-3 3
0.01% = 0.0001 = 1x10^-4 4
A photomask will typically have a base layer of chrome between 80 to 100nm thick (not including the chrome oxide on top of it) to achieve an optical density of 3.0 in the UV spectrum. For exposure wavelengths in the deep UV, such as 193nm, the balance is shifting towards thinner chrome (and lower optical density) in pursuit of better control of smaller features.