I'm not sure what "power-on hours" mean. It's obviously not MTBF. Is it max lifetime?
It's just that: how many hours it's designed to be turned on for. Compare to a lightbulb labeled to last for 1,000 hours but marketed as lasting for two years, with the fine print explaining "* when used for an hour per day". The expectation is that this particular drive will last for 24 calendar months, but that it won't be powered up and spinning the whole time. Imagine an office computer that gets turned off at night and weekends, and puts itself to sleep regularly throughout the day.
Given that this is a desktop-grade hard drive, that light duty cycle is probably not unreasonable. In any event, I'm sure Seagate has done their homework and 2,400 is the line between "we can reject warranty claims with 'you overworked it' as the reason" and "if we say our drive only lasts 2,000 hours, no one will buy it".
My biggest complaint with it is that there's no way that can responsibly listed as a NAS drive, but that's what their data sheet says it's good for. If you dropped that in a NAS, either it'd be spun up the whole time and burn through its specced lifetime in three months, or it'd be spinning up and down constantly and cause terrible performance with huge latencies on first reads after the drive falls asleep.