Certain areas, such as the entire country of South Korea, are legally forbidden from using GPS style location determination. They are afraid an invading army/missile will use it for targeting.
They put a lock on such services based on geography.
But the lock is not perfect, it uses a grid to determine which areas are GPS allowable and which are not.
The grid is supposed to exclude all of South Korea, but a small town happens to be just outside of their grid. So GPS devices work there.
Pokemon Go requires access to your GPS as part of the game (or rather, they designed the game to need access to your GPS so they can get your geolocation for advertising purposes).
As such, you can't play Pokemon Go in South Korea, EXCEPT in that one small village.