Anti-Aliasing: Antialiasing (AA) determines how jagged the outlines of objects appear to be. For a full description and screenshot comparisons, see
this page of my Gamer's Graphics & Display Settings Guide. It is recommended that you tick the 'Use application settings' box, so that the level of Antialiasing can be set within particular games and won't conflict with your Control Center settings. However
if you really want to force a particular level of AA in all 3D games, disable any in-game AA then untick the box here and use the slider here to force a particular level of Antialiasing. From 2x to 4x to 6x sample rate (or higher depending on your graphics card), the higher the level of Antialiasing, the less jagged the lines in games and 3D applications will appear, however the lower your performance will be. Note that Crossfire users will see more options here if Crossfire mode is enabled.