Writing original content does not mean that you have to write something that has never been written before by anyone else. You can say that, original content means writing in your own style rather than copying others style of writing.

Original content is something that was never said before i.e. content that was never published online. In reality though finding content that is considered original is very rare (at least for the average blogger or publisher) so what happens in the majority of the cases is publishing a different view or opinion for something that is already published.

What is important if you are republishing content is to make sure that your content is unique. By unique we mean that does not exist in exactly the same form anywhere on the web. One of the best ways to check your content's uniqueness is to use tools like copyscape.com that scan the web and find similarities in content. So, if your content passes copyscape with a score of 100% you at least know that it is not published in any other website online.

Orginal content means you can write a unique content with your  relevant knowledge of the particular topic