Alt Attributes, alt tags
alt attribute help Search Engines to understand the image they are crawling.
Alt stands for 'alternate'. It often called 'alt tag' but it is in fact an 'attribute'.
It is important to use alt attributes to describe your images to be in line with good SEO practices. Other image related optimization items are size & load time and image file name.
<img src="/image.jpg" alt="your-alt-attribute-here"/>