What is DPI or PPI?
DPI refers to Dots Per Inch in printing. The more dots you can input per square inch of material the higher quality printing you will see visually. PPI refers to Pixels Per Inch and is more commonly used on computers, but still refers to the quality of an image per physical inch. DPI and PPI are also commonly referred to as resolution. Images that use pixels, or dots, like this are known as raster images. Each tiny pixel is assigned a color and all the pixels together make up an entire image. The most commonly seen raster images are through digital cameras, scanners and monitors.
Because the resolution is directly related to the physical size of the image, if you take an image that is 1" x 1" at 300 dots per inch and then you scale up your image to 2" x 2" your dots in that inch just got spread over 2 inches, so 300dpi becomes 150dpi.