Based on the context clues of the RQ-170 description:
The much smaller RQ-170 is limited to 5-6 hr. of operation.
I'd say they're comparing the amount of flight time the drones can be in the air without having to return for refueling/recharging.
NM means nautical miles.
So, that means that the RQ-180 can stay up in the air for 24 hours at a distance of 1200 nm...
The Wikipedia article for the plane uses similar (but easier to understand) wording to the quote you've posted:
The RQ-180 is believed to be about the size of the Global Hawk, which weighs 32,250 lb (14,630 kg), and have similar capabilities of endurance (24 hours) and range (1,200 nmi (1,400 mi; 2,200 km)). This is much more than the RQ-170's endurance of 5-6 hours.
In general, the term on station is used to refer to the ability a mobile unit (generally aerial, space-going, or nautical) to maintain its position. The process of maintaining that position is called station keeping.