The rel=next and rel=prev tags are elements used to help search engines understand the relationship between paginated URLs in a series.
Generally, one URL should represent one theme or idea. If one theme or idea is split across multiple pages, search engines may become confused when indexing content.
However, there are many cases in which it makes sense for usability to split one theme or idea into several pages. Typical examples include product listings or search results pages with many items, or very large articles that have been split into two or more pages. The rel=next and rel=prev tags can help search engines understand the relationship between these pages.
The rel=next tag indicates that this is the next URL in the series, while the rel=previous indicates that this is the previous URL in the series. Therefore, the first page in the series will have a rel=next but no rel=prev, and the last page in the series will have a rel=prev but no rel=next.
Below is an example of an article that has been split into three pages: