What does DPI stand for?

DPI is an abbreviation of Dots Per Inch, or the number of dots that make up an image per inch (1 inch is 2.54 cm). So DPI is a way to indicate the resolution. Some programs call it pixels instead of dots (ppi), but it means the same thing.

How much DPI should my file have?

A DPI of 300 is required for the best quality printing, a lower number may result in blurry images. It is important to take into account that when you enlarge an image, the number of dots per inch decreases, and the image is therefore less sharp.