They are trying to explain that you can only measure as precise as your measuring tools and then you must estimate the last unit. For example, if you had a ruler with no inch marks only feet, you would have to estimate the length of an object that is say, 1ft 6in. If you had a ruler with inch marks, the same object would now be 1ft, 6.5inches. You simply measure as far as you can with your tool and estimate the next digit. So, the greater the accuracy, the greater the number of significant digits. That's how I understand it. I hope that helps and I hope its right.....LOL.