The resolution represents the amount of pixels or dots per inch (dpi).
The definition indicates the number of pixels in an image. When you buy a camera, it is the number of megapixels it has. The more you have a high-definition (a high number of megapixels) the bigger you can print.
To know the maximum size print of your photos, there is a very simple calculation to make. For example, if you want to print a photo 12''x18 '' at 300 dpi, you must (12x300) x (18x300) = 19'440'000 you must have a camera of 20 megapixel to get a perfect picture. It is possible to print at a lower resolution than 300 dpi, but be aware that if you go lower than 150 dpi, the quality of your image will be affected.