Post that again when you're not the top of the page.
It's used for having fluidness beyond the speed of either your logic or frame updates. Generally, your FPS is going to be quite variable, so you need to calculate exactly how much you want to move things based on how fast or slow the last update was. In variable timestep, you need to interpolate the logic because it is able to run at any speed. In the case of a fixed timestep, you need to interpolate the drawing, because the logic is locked into a certain speed but the drawing is not.