Much in the same way that the population of a country is, well, everybody in the country, a population in statistics is everybody in the group we care about measuring.
If we want to know what proportion of kids in Mrs. Dungey’s third grade class prefer eating Elmer’s glue sticks to off-brand glue sticks, our population is Mrs. Dungey’s third grade class...all twelve of them. If our goal is to figure out what percentage of American adults prefer Netflix to Hulu, our population is every adult in America, all 300 million-ish of them. If we need to know how many iPhone X owners wish they had a dedicated headphones jack, our population is every iPhone X owner...all however many of them there are (because Apple won’t say).