(NOTE: If you end up here after posting a comment, it is because of a bug in my journal. Most likely you timed out (the server forgot you). That in itself shouldn't be a problem but in some cases it screws up. It's on the to-fix list. Sorry.)

A captcha is a simple and stupid test to see if you are human or a spambot. The point being that if you are human the question/test is insanely simple, but for a computer or a script or whatever automatic spamtool, it just fills in something random (which is wrong.)

Read more about captchas at Wikipedia.