Low resolution means that the quality of your image lower than recommended. So if this design will be printed, the final print could appear blurry or pixelated due to the low quality of the file.
Technically speaking, images should be at least 300dpi for the correct print quality. You can also check this simply by opening/viewing your image on your computer and zooming into approximately 200%. If the image appears blurry, it will probably print this way too. You should then search for a better quality image for your design.
If you use an image from the free image gallery in our online design tool, it should always be printed in high resolution, regardless of the apparent image quality when creating the design. However, if your own uploaded image appears blurry, this could be an issue and should be checked carefully before approving your artwork.