In this context, POP would refer to Point of Presence, as noted in other answers.
More specifically, a POP is a physical location where servers are located.
Generally this is within a Data Centre, and most of the time it refers to dedicated hardware such as a leased server. Services with smaller requirements might be able to use a VPS as a POP, using something like Amazon Web Services or Digital Ocean as the supplier.
The exact placement depends on what your goal is, but normally companies try to place their servers very close to high bandwidth users and in a number of different countries.
If I had a DNS service running on servers in New York, London and Tokyo then I might refer to that as 3 POPs.
In such a case I would be placing them far apart to prevent any single failure from taking down more than one POP, and also to get DNS replies to the customer faster due to being physically closer to the servers.