What does OD mean?

OD means Optical Density. Basically, the higher the OD rating on the goggles, the less light will be allowed to pass through. OD3+ goggles are fine for lasers up to 125mWs. Further than that, you'll need goggles with progressively higher OD ratings. Specifically, the number is the power of ten reduction that the transmitted light of that wavelength is reduced. IE, OD3 passes 10-3 (1/1000) times the incident light.