It means Pixel Density of your screen.. Dots per inch
The lower it gets from your standard screen DPI value, the smaller the icons and things become in the UI..
Its like increasing the resolution of the screen like you do in your PC/Laptop Monitor Display..
320 is mostly the HD displays 1280x720
240 is i guess for a qhd res 540x960
and as far as I knw 160 is the DPI most tablets has..
You can alter it by an app like LCD denity changer/modder..
Hope It Helped, Hit thanks if it did