What is a "Postcode" and why does a survey ask for it?

Postcode, post code or postal code (commonly referred to as a zip code in the United States) is used for sorting mail into geographical zones within a city or town, however, Ireland's Eircode is unique to each residential or business address.

Postcodes can be in various formats that may include a series of letters or numbers or both combined, for some countries, this may also include spaces or punctuation. The postcode would be included in an address for the purpose of receiving mail (letters, bills magazines, etc.) to a person's home or their local postal services office.

Why do surveys ask for a postcode? 

Market research companies create targeted surveys in hopes of reaching specific demographics for their studies, location is one aspect of the demographics used as many surveys are location specific and require responses from residents of their target area.