reCAPTCHA is provided as a Google service, which provides the imagery, an audio alternative and the processing to determine valid entry in the form. Server Side processing behind this form gets the result from Google, and takes whatever action is appropriate.
See www.google.com/recaptcha for more information.
reCaptcha will display a table of images, some of which will depict everyday objects such as animals, crosswalks, traffic lights, buses etc. The user will be asked to select those images that contain a particular type of object.
If the images shown in the table are selected incorrectly, the message is not sent - though in practice there is some leeway: if any images are missed or incorrectly selected the process will repeat.
Note: This form does not submit any data to anywhere. It is for demonstration purposes only.