Antialiasing is a technique you can use to reduce the appearance of "jaggies" — the stair-step pixels used to draw any line that isn't exactly horizontal or vertical. In three-dimensional scenes, this artifact is most noticeable on the boundaries between polygons of different colors.
Direct3D supports two antialiasing techniques: edge antialiasing and general antialiasing. Which technique is best for your application depends on your requirements for performance and visual fidelity.