These are a few basic terms to help you get started:
Resolution is the measure of pixels in the display, usually expressed in measurements of width x height. For example, a monitor that is 1920 x 1080 is 1920 pixels across and 1080 pixels down.
DPI (dots per inch) is the number of dots in a printed inch. The more dots the higher the quality of the print (more sharpness and detail)
The actual size is best described with (you guessed it) actual numbers. Let's say you have a 5”x5” shape at 100dpi, but you need to make it a 10”x10”. If you simply scale it to twice its original size you go from 5”x5” to 10”x10” Since the size has now doubled, your resolution has been cut in half-that particular part of your graphic, is now 50dpi.
When you're submitting your print files, we ask that they're at least 150dpi and at actual size. The resolution of any file is only as good as the dimensions.
To learn more about DPI and what affects print quality, check out our DTG file guidelines.