pop. is a written short form of population — how many people live in a place. You see it in a fact box, on a map, or after a town name: Springfield (pop. 12,000). People almost never say “pop” out loud for this; they say “population.” The little dot is part of the written form. It belongs to guidebooks, encyclopedias, and signs, not to chat. Though actually, without the dot, pop is a completely different set of words — music, a father, a fizzy drink in some places, or a sudden sound — so the dot is doing real work.

人口（書面縮寫）