Accessibility - Alternative text
Alternative text or alt text is a short text description which is used if a visitor to your website can't see or display an image. It's used by visitors using a screen reader as well as browsers that do not support images or are on a slow connection. It's important to add this information to make your site more accessible.
Alt text is also used by search engine bots to help them understand the subject of your image, so adding this information can contribute to your search ranking.
Your alt text should be written for people rather than bots, so use full sentences instead of keywords. Some screen readers truncate long description, so you should keep the text short - around 100 characters maximum.
When writing alt text, think about the context of the image, as well as the subject. You can find some examples of alt text here.