what does 300 DPI mean ?

A 300 DPI (dots per inch) image means that every inch of the printed image will contain 300 dots of ink. This is a standard resolution for high-quality printing, producing crisp, clear, and detailed images, according to printing resources. 

You can check your image DPI by Image DPI Checker — Check the DPI of Images Instantly Online

You can also convert your image into higher DPI by using this free converter. Image DPI Converter — Change DPI of Images Instantly Online