Automated programs can't "read" the word you see in the box; only humans can. When you enter this word, it ensures that comment is from the genuine source and not from an automated spammer.

If you don't see an image, make sure your browser is set to display images and try again. If you're not sure about what the word is, make your best guess. If you're incorrect, you'll get another chance to enter a different word on the next screen.