The problem with many sites is that they have pages that are identical or nearly identical. This can happen when your site has many categories, paths, and dynamic data.
The solution is to use the canonical tag. The canonical tag is a tag used to tell search engines which version of a page is the desired or preferred version.
You want to be clear to search engines which version of the page they should index because if you have many versions of the same page then that page is unlikely to rank well at all.