Can someone give me a brief explanation?

In the lesson Basic HTML and HTML5: Add Images to Your Website, i saw this sentence

Note: If the image is purely decorative, using an empty alt attribute is a best practice.

Can someone tell me what does this mean ?

if you have an image that is just there to make your page look pretty, like a braid that countour your page, or a coloured line, or things like that, they serve a purpose only for sighted users. people that use screen readers don’t have any additional info from decorations, and if the image doesn’t load there is no loss of comprehension - so you can just use an empty alt to avoid having the image be read by a screen reader

The alt attribute specifies an alternate text for an image, if the image cannot be displayed, and that’s why alt attribute is also known by the name alternate text.

For some reason if a user can’t view the image on website (reasons like: if someone have slow internet connection, or if the user is using a screen reader etc). In this case, the alt attribute provides alternative informaton for an image.

See this screenshot:

the underlined text(Red color) it’s a alt text…due to src attribute mistype the image is not loading and alt text is being displayed in the place of the image. By reading the alt text a person can guess/undersatnd that what might be in the picture.

Hope my answer help you to understand.