nm stands for nanometer. It is simply the wavelength of the light. Sensitivity to specific wavelengths will depend on the optic in use. 850nm is the standard wavelength for IR; folks will use 940nm for reduced visible signature if using LED illuminators, though as noted in the above link, there is typically reduced performance due to reduced sensitivity to that wavelength by most devices.

mW is milliwatt, and is equivalent to lumens, which is to say, it is the raw output, and does not account for flood/throw. Note that laser outputs are much more efficient than LED outputs, which means that something like a DBAL-D2 with a 600 mW LED illuminator is somewhere between a full power AN/PEQ-15, which have a 30 mW laser illuminator, and a MAWL-C1+, which can go up to 67 mW.