It's great news for SEO though, because as long as you don't "stuff" your images full of keywords, it's a great way to add targeted keywords to your pages content.

Spiders eat alt tags up because while they can't "read" an image, they sure can read an alt tag.

A great example of an alt tag is if there was a picture of a brown dog with spots, you would have alt tags for it that said "brown dog with spots".