Post reach is the number of people who have seen your post. Your post counts as reaching someone when it's shown in News Feed. Figures are for the first 28 days after a post was created and include people viewing your post on desktop and mobile.
Facebook business pages allow you to view the reach of your posts and see how many people have engaged with your content by clicking on people reached, located at the bottom of each post.
By clicking on people reached, Facebook will also allow you to look at details of your post such as likes, comments, shares, and link clicks.