The RQ-180 is smaller than that concept, and it is not clear whether it will conduct strike missions. It is similar in size and endurance to the Global Hawk, which weighs 32,250 lb. and can stay on station for 24 hr. 1,200 nm from its base. The much smaller RQ-170 is limited to 5-6 hr. of operation.
What is the meaning of on station in the above paragraph? and I also don't understand the nm part in 24hr.1,200 nm.
"On station" means flying around the area of interest close enough to keep it under observation. The phrase you have asked about means "can fly 1200 nm from base to mission location, operate for 24 hours, and then return to base without running out of fuel." Another term is "loiter".
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.
