Complex sentence: In 3D the rotations and translations are also simply the possible moves of a rigid object, but the reflections are not; however, they can be imagined by thinking of looking at the object through a mirror.
Split into simpler sentences: In 3D the possible moves of a rigid object are rotations and translations. Reflections are "not" possible moves of a rigid object; however, they can be imagined by thinking of looking at the object through a mirror.