What exactly is Duplicate Content?
To put it short: Duplicate Content is content, that can be found online on more than a single website, meaning it can be accessed via multiple URLs.
Why is it bad?
It’s hard for search engines to determine which content is the best match for a specific search query. Should there be multiple websites with the same or very similar content, Google will make sure not to show all of these in its result pages. It will rather try to determine which page is the original source and thus granting it a higher relevancy. This can be bad news for a website, as all pages with duplicate content will have to share relevancy score and consequently will lose significance and ranking positions.