An isotropic material means a material which has identical values of a property in all directions. Glass and metals are examples of isotropic materials. In 3D printing this is important because the way you orient the part on the build platform could determine whether your part turns out to be strong or weak along a particular direction. 3D printing happens gradually in layers, not all at once like in the case of say injection molding or vaccum casting.
The Figure 4 production materials offer by 3D Systems have isotropic properties which allow users the freedom orient the part on the build platform so as to get better print quality, efficiency and speed without needing to worry whether the part orientation will affect part strength.