Resolution, also known as dpi (dot per inch) can be described as the number of dots that fit horizontally and vertically into a one-inch space. Generally, the more dots per inch, the more detail captured and the sharper the resulting image.
For an image to print properly, the image must be at least 300 dots per inch (300 dpi) at the final output size. If your file is not 300 dpi, you can not simply increase the dpi from a low resolution to a higher one by increasing the dpi in your imaging program.