It refers to the use of breadcrumb trails on a website to improve the site structure for search engines. By using breadcrumb navigation, a website can indicate the hierarchy of the site and the relationships between its different sections. This can improve the visibility of the site in search engine results pages (SERPs) and make it easier for users to find the content they are looking for. Additionally, it makes the website more user-friendly by showing the user where they are on the website and how to navigate back to other sections.