In this example below, the image has no alt attribute in the img html tag. Reading this image from a screen reader will provide no image description but simply tell the user an image is present.
The screen reader will read "Image".
In this example below, the image has an alt attribute in the img html tag which contains an accurate description of what the image is. Reading this image from a screen reader will tell the user an image is present and read this description so they can understand what the image is of. In some older browsers, the content of the alt attribute may also appear as tool tip when pointing the mouse at the image.
The screen reader will read "Photo of Mt Fuji as viewed from Lake Asahi. Image".