When you Add a Page in the Pages Panel you can choose Page Builder as your Page Type. This allows you to set up a page layout using Designed Sections (see linked article for more details). This tool and special mode are for rapidly placing predesigned sections together into a new page to create a layout. Then adding the page to your site pages for content editing. A great tool for special pages and occasions.
In general, it is recommended that new pages are created by duplicating existing pages you already have built. This is to help keep the website layout and design consistent. It also saves time, as you would be using a page layout that was already built for your school's website. Only use the Page Builder or other Page Types for special pages or circumstances.
Once you have selected Page Builder you will launch into Page Builder Mode. While in Page Builder Mode you will see "Page Builder" in grey in the upper right corner and your left panel will now be available Designed Sections. This mode is not for editing the content, just for choosing multiple Designed Sections.
You can rename the page layout you are working on in the upper left by click the 3 Dots
Your can Save For Later in the upper right corner. Page Builder pages are saved in the Pages Panel under Pages in Progress.
Once your are done, you click Add Page to Site. The page will now join all your other pages in the Pages Panel like normal (new pages always appear at the bottom).