Dots Per Inch (dpi) refers to how many solid dots of colour are printed within a square inch. Photographs are a good example of how lots of solid dots of different colours make an image. The more dots, the more detail is visible.
Using a magnifying glass, you can sometimes see these individual dots, particularly on newspapers, which are produced at a lower dpi.
Designers work to 300dpi for brochures and flyers that are viewed closely, where detail is key. Large-format printing, such as banners and exhibition printing, can be produced at 150dpi or 72dpi as they are viewed from a distance, so the loss of detail from larger dots isn't as noticeable.
Different production methods print at different dot-per-inch densities. However, the added detail isn't realised if the supplied artwork is low resolution.
A photograph at 150dpi, placed on a file set to 300dpi will simply result in one dot spread across multiple.
