Chart of Cumulative Percent Defective

Felipe,
The chart shows you that at the start of your sample size there is greater variation in the prediction of what the true percent defective is.  As your data collection grows you should see the chart level out to a fairly steady number and this tells you that over a larger sample size you are seeing (predicting) a true percentage defective.  So if you have a process that produces about 88% good parts and you sample 1000 pieces as you near 1000 you should see that chart level out to around 12% hence you could be fairly accurate in predicting a % defective of about 12%.  The chart also provides some confidence boundaries at 95% confidence of where the true % defective actually lies.