The alt attribute defines an alternative text description of the image. That's helpful for a software such as search engines or users who can't see an image. In short, it's a really important thing. The common problem is how to define it correctly. I have some tips that help make it.
In most cases you have to describe what an image represents. For example, Monet's painting of a poppy field, completed in 1873:
<img src="picture.jpg" alt="A poppy field in Sunny weather where several people walk">
If the image contains some data it can be described in the alt attribute.
<img src="piechart.gif" alt="The pie chart: The percentage of each browsers - Internet Explorer 22.54%, Firefox 19.26%, Safari 15.59%, Android 4.59%, Opera 4.53%, Others 4.46%, Chrome 29.03%">
If an image doesn't affect on information and it doesn't provide to users helpful information then leave the alt empty.
<img src="picture.jpg" alt>