DPI is dots per inch
Basically you want 72-96 DPI for images being viewed on the web and 300 DPI is best for print.
If you find an image on google images and want to find out the DPI you can save the image and right click on the image in your folder. Go to properties then details and you should be able to find the DPI there. You do not add the DPI together the Horizontal and vertical DPI is the total, If it says Horizontal DPI 300 and Vertical DPI 300 your total DPI is 300