What is a buffer?
A buffer is an area that surrounds one or more map features. The size of a buffer can be a specified width (e.g., 5 miles from a store, 1000 feet from a school, 1km from a highway) or variable based on some attribute (e.g., contains 10,000 households).
With a GIS you can build buffers and estimate the demographic attributes for those buffers. For example, you could find out the population and average income of people that live within 5 miles of a site that you are considering for a store, or you can find the number of people within 1/2 mile of a proposed bus route and what their average car ownership is.