On most sites there is content that appears in a number of places. For example, contact information might appear in the footer of every page and on the contact page.
To save your editors needing to duplicate that content, you can set content as being shared by checking the Share across all pages checkbox in Region Options.
Once you have done this the Region displays at the top of the content listing in Perch, so that content editors know that changing it in one place will change it everywhere.
To reuse the Region in your pages simply add a Region with the same name as the Region you have just shared to any page you would like it to appear on. This process is demonstrated in our video about Shared Regions.