Aliasing is the staircase effect that occurs when diagonal or curved lines or borders are drawn on raster displays consisting of square or rectangular pixels.
Pyramid is aliased on left, antialiased on right.
Antialiasing improves image quality by smoothing such internal image edges. Antialiasing can be either on or off. Turn this off only when you're rendering test images and want greater speed. Leave it on at all other times.
You can also turn antialiasing off for the Material Editor sample slots to speed up redraw of the sample objects. Click the Options button in the Material Editor and turn on the Antialias toggle on the Material Editor Options dialog. Default=off.
Tip: To control whether a background image is affected by the renderer's antialiasing filter, go to Customize 
Preferences Rendering and toggle Filter Background in the Background group.
Preferences Rendering and toggle Filter Background in the Background group.