From the official site, CAPTCHA stands for Completely Automated Public Turing Test to Tell Computers and Humans Apart. If you drop the “to” and “and”, shouldn’t it read CAPTTTCHA instead?
CAPTCHAs are those images you see with a phrase in them. The user has to type in the phrase they see in the image and the script upon submission will compare the user phrase with the saved phrase. If the two match then the submission goes through; if not, the user is booted back to the form usually with an error message.
The reason for the use of CAPTCHA images is to tell computers and humans apart as the meaning says. Computers have a hard time determining the phrase because of the other lines criss-crossing over the letters, so scripts can’t be run on the comment form.