Modifier override allows you to enable or disable modifiers of objects while rendering . This makes it possible to include intensive modifiers such as Hair and Fur, only at render time, and increase productivity in the viewport. Either using modifier Xform transform an object in a certain way in saparate pass.
As an example, one object may have three modifiers: Turbosmooth, Displace and VrayDisplacementMod. When working in the viewport, you keep all three on to see how displace looks like. However, when rendering, Turbosmooth and Displace turn off and VrayDisplacementMod works correctly and on rendered image you see roughly the same as in the viewport.