Well it sure is unfortunate that we can't post videos on this forum because it'd be easier to just do that and you'd see exactly what it means.
"side to side motion" means first the oar goes in one direction towards one side, and then it goes the other direction to the other side. If you're standing at the stern of the boat, looking "backwards", then the oar would go to the left side, then to the right side, and so on.
"reversing the blade" would mean that when you go one direction the blade is held at a certain angle, and when you "turn", meaning switching direction, you reverse the angle. So if one angle of the blade moves you forward when you move the blade to the left, then in order to move forward when you move the blade right you'll have to reverse that angle.
Again, posting a youtube link would have sufficed. Or video. But alas my overly wordy explanation hopefully suffices.