According to Google’s Gary Illyes, near duplicate content is a piece of content that was slightly changed:
Think of it as a piece of content that was slightly changed, or if it was copied 1:1 but the boilerplate is different— Gary Illyes ᕕ( ᐛ )ᕗ (@methode) 11. Juni 2017
If you post pieces of content from other websites on your own website, this is near duplicate content for Google. It is also near duplicate content if you use all of the content from another website with different boilerplate content (headers, footers, navigational elements, etc.).
How to check your web pages for duplicate content
Among many other things, the website audit tool in SEOprofiler checks your web pages for duplicate content. The website audit tool in SEOprofiler makes sure that your pages do not contain errors: