It is a simple sum.
It refers to the output voltage when an output is HIGH (\$V_{OH}\$) as a minimum (it's in the MIN column) of 2 volts below \$V_{CC}\$, so if \$V_{CC} = 5V\$ then \$V_{OH}\$ will be at least 3V.
For CMOS you will often see something like \$0.6V_{CC}\$, where the value is 60% of the \$V_{CC}\$ value.
A rail-to-rail output amplifier is designed to swing very close to the supply rails, unlike older op amps which sometimes couldn't get closer than a few volts.
However, there is a finite output resistance in the output stage, meaning you can't swing exactly to the rails. As the output stage starts to deliver current to a load the voltage drop across the output devices has to increase.
What the datasheet is saying is that with a 10K resistor between the output and ground, you are guaranteed to get within the specified voltage of the voltage rail on the output before saturating or clipping.
Lower resistance will result in a higher drop.
Best Answer
NOM = Nominal, that's the value you normally can expect, and what the device is designed to.
Note that nominal values are often not the best to calculate with. If you want to go worst case you'll have to calculate with Minimum or Maximum, depending on the parameter and the application.