Captcha is a genre of tools that are used to differentiate between human and machine input. This is done by presenting a challenge that is easy for humans to solve, but difficult for machines. The most common form of captcha is the image-based captcha, where the user is presented with an image containing some text, and is asked to type the text into a text box. The idea is that a human can easily read the text in the image, but a machine would have a hard time doing so.

(This is also known, more colloquially, as "the thing that makes you type in a bunch of squiggly letters to prove you're not a robot.")

For more information, see this lecture by John Mulaney.