Studying about HTML5 I came across the alt property being used in the <img /> tag.
Example:
<img src="https://s-media-cache-ak0.pinimg.com/736x/9e/fd/27/9efd27afef4e8127923fbce92b8c967d--minions-minions-funny-minion.jpg" alt='Essa é uma imagem sobre Minions' /> I'd like to know what the alt purpose is in a <img /> tag, since nothing visual is displayed.
It is to put the description of what the image contains. It will be shown in place of the image if it can not be displayed. Useful for:
The latter is important because reading software for those who are visually impaired can read what is written and "speak" to the person. There is no way to "tell" the person what the image is, you can only do it with a description.
Many people think that everything will always work, but there are several reasons why the image should not be displayed:
Do not use space to put the name of the image or something less useful. The text should describe the image as best as possible to meet these requirements stated above. If necessary you can put a long text, but this is not always necessary, you do not need to make an artistic critique of the image :). If there is conflict between the goals, do your best to address the most important and try to help the others.
Do not use if the image is merely decorative, use <alt = ""> in these cases, only images important to the general context of the page will require this. Be sure to use this form so that the browser does not show anything, so you do not try to describe it as having an image there, which in fact would only be useful for the page rendered completely.
The appropriate example is what is in the question itself :) It does not have much secrecy.
Here on the site you can do this when you add an image and few do, that's bad.
In my opinion, the great importance of alt is in accessibility, including W3C ( World Wide Web Consortium ) has this slide about accessibility on the web.
There are some screen readers like ORCA, for Linux and JAMS for Windows, in this site > has an explanation about them. With the tag alt these softwares inform disabled users what is in the image. This is why the main function of the tag is to provide an alternative description for the image.
If you are interested in learning more about accessibility on the web, I recommend reading book from W3C.
Remember that small details such as <img src="https://goo.gl/3pS3AU" alt='Essa é uma imagem sobre Minions' /> in your HTML make a big difference for disabled users.