Many times images are not perfectly aligned with the grid that you're interested in, or the image is slightly distorted. We put this tilt in intentionally, but skew is often the case with air photos and satellite imagery. These images often need some kind of geometric correction to make them compatible with an accurate map of the same area. Like most software for GIS and remotely-sensed data, Idrisi has tools for correcting images. Resample allows us to rotate images or do minor changes in projection.
Rubber sheeting is good description for the transformation.
Imagine the image printed on a rubber sheet, which you could then turn and stretch in various ways. We basically only need to rotate our image. Rubber sheeting can correct more serious defects, although not extreme ones. You may not understand all its concepts, particularly if you haven't used remotely-sensed data. But you should get a general feel for what's going on in this section.