This is a security test to determine whether or not the user is human. This will ensure that only human visitors will actually submit the form rather than spamming robots. It is well known that programmers can write programs that visit many sites on the Internet, by following links from page to page. They can collect emails listed on those websites, submit contact forms, or even try to break passwords by trying to login over and over again with computer generated passwords.
Fortunately, it is impossible for a non-human (e.g. a computer program) to extract a word, or a combination of letters and numbers, from an image if the letters and numbers are not all of the same size and of similar font. This is why, the Image Verification security method ensures that only real people can actually submit the form.