The Domains tab lets you put your app on your own web address instead of a shared link.
A custom domain makes your app feel like your own product. Instead of a shared link, visitors see your brand, for example app.yourbusiness.com. This builds trust and looks professional.
You need to own the domain: Buy the domain first from any registrar, then connect it here. If you do not have one yet, you can launch on the shared link and add a custom domain later.