Word Verification

What is this "word verification" we have whenever we post a comment? I've been catching up on a few blogs (and finding a few new ones). Every time I post a comment I have to verify the jumbled, italic, strange looking letters before my comment will be accepted. Will one of my more experienced blogging friends please explain to me the purpose for this nonsense? There are times when I can't tell what the letter is supposed to be. So the blogger "security" changes the "word" and I type in the new word. Why? I'm beginning to think I am dyslexic trying to figure out those strange figures they call letters. Just my brain rattling for the evening.