AI art is art generated with the help of artificial intelligence. The No AI tag is an HTML tag added to your website's header section. This tag is meant to alert the people in charge of AI systems/programs that you do not want your content to be used on any systems to generate AI art or be used as training material.
Here's an example of the No AI tag, which you may also check by inspecting your site:
Why Portfoliobox chose to add a No AI tag to all websites?
Portfoliobox has added "No AI tags" to all websites to protect our users' content from being used as material for AI systems. Please keep in mind that this tag will not wholly prevent robots from crawling your site, but it will warn them that you do not permit them to use any of your site's content on their AI system.