If you're just getting started with your website, but you want to give your customers a place to go so they know you'll have a website up soon, you can set up a "Coming Soon" landing page, also known as a construction page.
Anytime you change your theme you will lose all of your content. When you publish your Website using a new theme and content, the Coming Soon page—and any other Website content you may have previously added— will be replaced.