PPI, or pixels per inch, is first and foremost a simply measurement that’s dependent on a screen’s size and resolution. However, it’s also a value written into the code of an Android device which essentially controls how large interface elements on the screen. While you obviously can’t stuff more pixels into an existing screen, you can change this value in the code, assuming you’re rooted. There are several apps to do this, where the free DPI Changer app is an obvious choice because it’s open source and free. It shows you your current DPI (PPI) setting, allows you to change it, and then tell you to reboot for the changes to take effect.
I changed the PPI or my Galaxy Tab Plus a few times until I landed on 140 PPI as a nice change from the default 160 PPI setting. This scaled the entire UI on the tablet down, giving the illusion of being a larger screen than it really is. The tablet now things that it’s a 1024 x 600 resolution on a 8.5-inch screen rather on a 7-inch screen, making interface elements smaller in resolution to compensate for the screen size making them physically larger.
The point of this is to free up space on the screen. With interface elements being smaller, everything else has more room. The difference between 160 and 140 DPI isn’t extreme, but enough that you clearly see the difference in the screenshot above, where the 160 DPI option is on the left and the 140 DPI option on the right. You notice how the album art is larger for 140 DPI, which is a result of the content area being bigger due to smaller UI elements. Anything that has a toolbar and a content area will end up benefiting from this to some extent. With everything being smaller you obviously also have to be more accurate with your finger presses, which is why you shouldn’t change it too much. Incidentally, if you adjust the DPI up enough, the OS will eventually start thinking it’s running on a phone and remove the button bar and display the phone status bar instead. As with everything root, use at your own risk.