After falling down a pretty deep internet rabbit hole - It seems that its the polarity of the sync signals - in this case a VESA Signal 1680 x 1050 @ 60 Hz timing. These seem unique to a specific set of refresh-rate/resolution/standards, and arn't of particular use to the end user. They're handy if you wanted to make a microcontroller (say in a toaster) output VGA I suppose, or had to set up some interface by hand.
The NP listed in your monitor's menu setting option stands for Negative/Positive input signal polarity. Other input signal polarity settings that you'll see are NN and PP (for Negative/Negative and Positive/Positive, respectively).
Hobbs has it right. The horizontal sync pulse is negative and the vertical sync pulse is positive for this Samsung monitor at this resolution. You really need to talk to a color TV repairman from the 1960s to understand what is going on because all of this is holdover standards from the days of analog displays. The horizontal sync pulse ends the sweep of one line, and the analog TV shuts off the electron gun that fires the electrons at the screen to make the picture until, the sweep coils are ready for the next sweep one line lower. (The electron paths are magnetically bent after coming from the electron gun (Cathode) to the proper place on the screen.) (If the coils fail, the electrons come straight out, and there is only a dot in the center of the screen.) When the last line is finished, the vertical sync pulse occurs and the electron gun is shut off until the coils are ready for the next set of sweeps. The sweeps can be one line after the other, or interlaced. (Where only the odd lines are scanned, then a vertical sync pulse, then only the even lines are scanned.) Please note that two different meanings of the word scanned were used in the 1960s, and I have maintained this convention for this conversation.
