When creating designs for print, you need to create a 300dpi resolution document. This is the standard resolution for printing as it creates a nice, crisp, clear print. The lower the resolution of an image, the worse the quality of the print will be, for example printing a 72dpi image will produce a low quality, pixelated print.
Any higher than 300dpi will print well but you can’t see the difference. For example, printing a 600dpi document will produce the same result as a 300dpi document, but your image file size in megabytes will be larger. Having an unnecessarily large file size results in slower upload and download speeds and means the printer's software takes longer to process the image, causing delays in production.
Sometimes we allow a print document at 150dpi. This is usually the case when working with wide format graphics that will only be seen from a long distance (such as PVC banners), so the close up quality doesn’t matter so much. The lower resolution makes life easier for your computer and the printers, and results in a much smaller file size.
You can check the dpi of your image in Photoshop. Go to 'Image' in the menu bar at the top and then select 'Image Size'
You can then see the resolution as per the image below.