In general, "chamfer" refers to rounding off a sharp edge or corner. 3ds Max includes two 3D primitives that incorporate chamfering: ChamferBox and ChamferCyl. In addition, chamfer options are available in various other program features. To find these, check the chamfer entry in the index of this manual.
In the context of NURBS, a chamfer curve is a dependent object that is a line segment that connects two curves. It is controlled by the curves it connects, and by two length parameters that control the distance back from the point of apparent intersection of the "parent" curves.
NURBS chamfer curves