what does this mean?

It's an attempt to represent how light-sensitive a particular camera is. F8 @ 2000 lux would be half as sensitive as F11 @ 2000 lux.
 
This expression (+S/N ratio) is the standard way professional video cam's light sensitivity is specified The representation is somtimes being taken over for (pro)consumer cam sensitivity specification, but the related S/N ratio is then often omitted. This omission makes the spec fully worthless. All cams can get a reference gain which can result in an impressive sensitivy figure...at the expense of noise, even under normal light conditions
See also http://www.dvinfo.net/conf/showthread.php?t=12440
 
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