Captcha is for defining whether your traffic is human or bot generated. If your visitors solve captchas with success, your traffic gets high quality scores and costs more.
100% maximum captcha rate number means that you agree to show captcha to 100% of your visitors if necessary. Usually it's shown after maybe 1 click of 100 - if everything is okay, there's plenty of clicks and visitors who see captcha solve it successfully. If your visitor fails to solve captcha, your ad unit is punished with money, the next visitor also sees captcha, if he solves it - your traffic is regarded as bot generated nomore.
If you've got 50% maximum captcha rate, we cannot show captcha to the next visitor after a fail. You'll wait until the next (second) unique click to prove your visitors are not bots. If you've got 0% captcha rate, we cannot define % of human visitors on your site at all and regard your traffic as suspicious.
So actually 100% is the best choice.