I would say it was the opposite of 'hidden' text - it's primary function is to describe a graphic in situations where the graphic can't be seen.
Search engine bot can't read the image so it is important to name your images especially if you have plenty of it in your page.
No the image alt tag considered as in text. Because Google has no eye that's why SEO's describe images as in alt tag
Alt tags are for spoon feeding the search engines crawlers. You can't be penalized for helping them see what they can't with their limited capabilities.
The alt attribute of the image tag is not hidden text. It is used to label or describe the image. The text appears in place of the image if images are disabled in the browser. You should use it to label or describe the image and you can place keywords in it if they are related to the image.