What does "Duplicate Content" mean?

Duplicate Content:

Duplicate Content is when you have a lot of content on your site that is identical (or very similar) from one page to another. Duplicate Content can be created by having separate versions of pages for mobile and desktop displays, with different formatting but virtually identical text and photos. Or it can be done by creating printer-friendly versions of all your pages. Avoid duplicate content. For the most part, Google ignores it, but Google might ignore the wrong pages! In a perfect world, if you took content from another website and posted it on one of your web pages, Google would simply ignore the content of your web page. Affiliate websites do this regularly, and then wonder why their websites don't rank well at Google. Duplicate content issues also arise if you add variables to a URL so you can track users as they move through your website. The way to handle Duplicate Content issues is to have proper Canonicalization of pages, so your content isn't ignored. And don't put up multiple versions of the same information.

 Google Dev: Avoid Duplicate Content