From Mac Guides
Resolution Independence is a feature which is partially functional in Tiger and Mac OS X 10.5 Leopard (see Changing the effective screen resolution). While originally planned for full release in Leopard, inconsistencies in the UI means that you can only enable it in the Quartz Debug application (included in Apple's developer tools). It is unknown whether Apple will allow all users to access resolution independence in a future update to Leopard. It allows Apple to use very high resolution displays in their computers without impairing viewability for the majority of users.
As display technology becomes more advanced, manufacturers can include more pixels per inch in their displays meaning that the display resolution can get even closer to the quality seen on glossy magazines and high-quality printers. However, higher resolution on a smaller display means smaller fonts and a smaller User Interface. Not only is this a problem for those with weaker eyesight, but it also makes it harder to click certain aspects of the display such as buttons as they also appear smaller. This is because the operating system assumes everyone is working with the same pixel density which isn't true. Resolution Independence fixes this problem by allowing you to "scale" the interface to a size that suits you. Making the interface "larger" this way means that even more pixels are used in the drawing of the interface. To avoid distortion vector graphics or very high resolution bitmap graphics are used.
Currently, the highest pixel density in a Mac is the 11.6" MacBook Air at 1366*768 with 135 ppi (pixels per inch).