What does "tested driver" mean?

Tested at the end of the driver name in the Additional Drivers utility means the driver is considered to be first class in terms of stability and reliability. Other drivers that are not tested could be considered to be second class by comparison.

The reason why there are different grades of graphics card drivers is that for newer models of graphics cards the tested graphics card driver does not always work properly. The same thing goes for very old graphics cards that only work properly with either a legacy graphics card driver or the default built-in open source graphics card driver. Offering a variety of graphics card drivers improves the chances of the user finding a graphics card that works properly on his system.