Reach is the number of people your post was shown to. Reach can happen as a result of paid adverts or for free. Organic reach is reach that's not as a result of adverts.

To understand how to increase the organic reach of your posts, you need to understand how Facebook decides how much reach a page post should have and how this has changed recently.

These articles from Facebook explain this:

https://www.facebook.com/business/news/update-to-facebook-news-feed

https://www.facebook.com/business/news/Organic-Reach-on-Facebook

The formula Facebook uses to work out how much reach a post should have is sometimes called 'the algorithm' and used to be called EdgeRank.