A population, in statistics and other areas of mathematics, is a discrete group of people, animals or things that can be identified by at least one common characteristic for the purposes of data collection and analysis.
To gather information about a large population, data is usually gathered from a sample. For example, if market researchers wanted to get representative numbers for Millennial mobile phone users in New York City (the population) who had no landline telephone, they might randomly query a small percentage of subscribers to major carrier services within the city that were born between 1980 and 2004.
Although the researchers would not have a precise number, as long as the sample is large enough and the study adequately controlled, they should have a number that gives them a fairly good idea of the prevalence of fixed-mobile substitution among that demographic.
See an introduction to populations, sampling and parameters in statistics: