These are those words that are distorted by lines or squiggles on web pages. You are asked to type out the word correctly on your keyboard to enter a secure internet site. Usually it is easy to see what the word is amidst all the other lines but sometimes the word gets lost in the mess. It has something to do with entering a secure site but all I can figure is that it prevents people with bad vision from accessing certain web pages.
How do these things work?
They are intended to prevent automated systems from accessing whatever the system is protecting. It’s called CAPTCHA, and works on the principle that human pattern recognition is vastly superior to any OCR system yet designed; we can pick out letters and numbers on a “noisy” background easily, whereas computer programs cannot.
It’s called a captcha. It’s not about entering a secure site. It’s for keeping bots from spamming web-based forms.
Until I read that wiki article a few months ago, I had no idea that CAPTCHA was an acronym. 
Captcha ya!