Captcha is when you have to decipher weirdly written words to continue with a webiste. It could be for joining a site or just making a comment on a blog. I deal with spam on my blog. Blogger is now pretty good at sorting the chaff from the weeds. I hadn't checked my spam comment folder for over a week and there was about fifty spam comments there, none that should not have been marked as spam.
If you want people to comment on your blog, then I suggest you turn captcha off. I am experienced at captcha but it took me two attempts to log my comment on Hels' blog and seven tries to log a comment on Ad Rads' blog. Twice I can wear but seven???? It is not like I said anything important, just a comment, but I am over it and I am not doing it again, no matter the cost of disconnection with blogmates.