Generally, Captchas provide information which the user has to interpret. The given information is designed to be illegible by computers, so that only humans can give an interpretation and be confirmed as such.
Apart from often being a major user annoyance, these perception-based tests pose a problem to blind or visually impaired users: They might not be able to solve the visual test and cannot rely on assistive technology like screen readers, since Captchas are designed to not be machine-readable.
Today, the widely used reCaptcha test developed by Google simply involves clicking a checkbox saying “I’m not a robot”. The Captcha tracks mouse movement and browser data to verify the user and has improved the ease of use immensely. But when the initial test fails, an image selection form is prompted, still.