Photos that have too low a resolution for the chosen size of the print will look blurry and jaggy.
From the gadgetwise.blogs.nytimes.com
When you do that, you discover that close up, things look jaggy on a large 23-inch screen with 1920 by 1080 pixel resolution.
From the nzherald.co.nz
Because of their fixed resolution, printing bitmap images can easily produce grainy, jaggy, or blurry results if the resolution is not ideally suited to the printer resolution.