If you have multiple pages in a website , say , home page , about page and contact page , now all common content that is not changing from page to like navbar , header and footer you place them in a template page you can call it layout.html , and in this template page layout .html you define a reserved space for the content that will be changed from page to page using an empty block tag ...
Now for example lets create home page , first you extend the layout.html , then you put inside the block tag the data specific for home page only ..
You see , the purpose of this system is not to repeat the static content in navbar , header and footer in each page