Thank you for the opportunity to help you with your question!
Say, for instance, you want to know what type of car people over 55 prefer to drive in the United States. Since it would be impractical to ask every person in the United States that question, you take a survey of the people over 55 in a particular community that is not too large. So, you mail a survey to the people over 55 in AnyTown, USA and you use that to represent the larger population in the entire country.
Please let me know if you need any clarification. I'm always happy to answer your questions.