Layouts serve as templates for the pages of your site.
Define recurring elements like header and footer once in the layout to have them show up on all pages. Use page blocks for elements you only want to use on certain pages. Any elements you set up here you’ll be able to use as needed later in pages.
Use styles to change the look of elements in layout mode. For example, change the size of your headlines with a font style. In layout mode you can specify which modules should be editable in page mode. When you create a page from that layout, all elements outside of the page block area are already there.
Now fill each page with the page blocks you defined earlier. This lets you build out pages with your own custom building blocks. Build as many pages as you want from only one layout.