Post reach is simply how many times a post has been viewed on Facebook.
Post engagement involves more than just views. It is recognized as any sort of interaction with the post. This includes clicks, likes, comments and shares.
Why does that even matter?
It is important because your stats can show high reaches, which means it lands on someones page and they scroll by it, but you don't necessarily grow your page with that. A view doesn't let us know if the ad is working or not. Engagement gives more insight on how the ad is compelling the viewer to interact with your page.
How can we use this information?
Know your Facebook insights will allow you to make changes with your posts. You can adapt to what is working and what isn't really serving your page. This will allow you to head in the right direction of growing you page with your followers.