In reversing the shape they mean, where it said to [for example] decrease two stitches on the right side of the knitting, they now mean decrease two stitches on the left side of the knitting.
Try to picture in your head looking at a sweater/cardigan. You are now essentially repeating the first side you made but making a near mirror image.
Does that help? If not, I'll stay on a bit longer and see if you need more explanation!