#'s Workshop

[Kitsu]

From the "Spriting Dictionary"

ANTI-ALIASING (aka AA)- It's a technique where you place mid-tone pixels in strategical places to make the line smoother.

In this case, the black line is anti-aliased by gray pixels placed on the 'corners' because the background is white (white+black=gray). If the line was red, then the gray dots should be changed to light pink to properly anti-aliase it.
Keep in mind that that image example was generated with Photoshop, so there's a lot of midtone pixels aliasing it. In spriting, obviously you'll want to use a less extreme AA. Only place them in jagged places; if the line is already smooth without AA, there's no need to do it.

You should go read that topic, it could be useful.