The Full Form Of POP is a Point of Presence Or Post Office Protocol.
POP protocol
In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by email clients to retrieve messages from a mail server. The current version of POP that is currently still used in POP3 (POP version 3), though IMAP is the more used email protocol.
POP protocol provides access via internet network for a user client application to a mailbox maintained on a mail server. So the messages come to the server and then when user opens his client application, the messages are downloaded to his device. The messages are then deleted on the server usually.
Point of Presence
A point of presence (PoP) is an artificial demarcation point (which originally in telephony meant point at which the public switched telephone network ends and connects with the customer's on-premises wiring) or network interface point between communicating entities. A PoP typically houses servers, routers, network switches, multiplexers, and other network interface equipment, and is typically located in a data center. A common example is an ISP point of presence, which typically houses numerous PoPs.