PoP stands for point of presence, and it refers to the physical presence inside a location (usually data center) where multiple connected devices or networks share a connection and space.
Perhaps, most people relate PoP to the ISP PoPs (Internet service provider points of presence). It is the access point through the one users can connect to the Internet of their corresponding ISP. Big networks usually have many more PoPs.
PoPs usually involve different interface hardware necessary to operate (routers, switches, servers, multiplexers, etc.), and they are physically located in data centers.
Talking specifically of DNS PoP (Domain Name System Point of Presence), it is every spot where the DNS provider has a DNS server or servers, and users can set an authoritative name server for their domain name. The user can choose DNS PoPs included in the DNS provider’s network (different locations) to establish their CDN (Content Delivery Network).