DPI (Dots Per Inch)
This is a printer term. It refers to ink dots on paper.
PPI (Pixels Per Inch)
This is the digital term. If you have an image that is 1000 pixels wide, and you print it at 10 inches wide, that is 100 PPI. That will look blocky.
The Golden Rule
Aim for 300 PPI at final print size. If your image is 3000 pixels wide, you can print it up to 10 inches wide safely.