You can set a splash page by going to Website > Pages > Select Page > Page settings.
Then, check the box next to "Make this the splash page" and click Save page. Things like the top banner, navigation, sidebars, and footer are taken out, and instead only the content of the page displays with a big "Continue to [page name] page" button added to the bottom.
Splash pages will show to a visitor only once. The most popular page-types used are the signup and volunteer pages. Your homepage cannot be set as the splash page and a page must be published in order to see the option to make it a splash page.
If you've created a custom design you will want to edit the splash.html file, located under Websites > Theme > Current Custom Theme > Templates > splash.html.
To see what the page will look like in testing, you can manually trigger splash page mode by adding "?splash=1" to the end of any NationBuilder URL.
For example, here's what it would look like if I made my signup page my splash page (note "Continue to [Organization name]" at the bottom):