Here's a simple explanation: parallel fifths and octaves are forbidden. If the interval between two voices in one chord is a fifth or octave, and in the next chord after is the same, that's not allowed. As a bonus, my theory professor gave us this awesome way to check for them: find every fifth or octave in your writing and check the notes that proceed them to see if the previous interval was the same. This is a lot better than checking soprano against alto, then soprano against tenor, etc... which is what I used to do, because I am a very slow theory major. The reasoning I was taught behind this rule is that in early counterpoint a sense of independence was valued between the parts. This independence is still maintained in the sense of parallel thirds and sixths, but with octaves and fifths the voices tend to lose their individual character. And I'm going to assume your book is Kostka and Payne? Classic... They were using that when my chorus teacher went to school, and they used it with me too.