Images might be unavailable for a number of reasons (it might not exist, it might not be named properly, or the file path in the code might be incorrect). These images are marked as broken.
Img alt tag text
Alternative text (common shorthand name 'alt tag') is a text alternative of the image for screen reader users. Alt tags will be displayed in place of an image if an image file cannot be loaded.
Screen readers for the blind and visually impaired will read out this text and thus make your image accessible.
Furthermore, Alt tags provide better image descriptions to search engine crawlers, helping them to index an image properly.
img src="nikola-tesla-portrait.png" alt="Nikola Tesla portrait"
img alt tag text is mandatory. If missing, it will be highlighted with red by Website SEO Audit Tool.