A domain name is the virtual address of your website. Ours is websitebuilderexpert.com. That’s where you find us. The New York Times’ is nytimes.com. That’s where you find them. And so on. Your site needs one too, and when setting up a WordPress site it’s something you may have to take care of yourself. Bluehost lets you choose a domain for free as part of the signup process.
You can absolutely do that. If you want to upgrade to a paid plan to get some of the features you want, then you can switch at any time through the My Products option in your account or with the help of a GoDaddy Guide. Once you upgrade your site will have all the same customizations and work that you've already put in place, just with the added features.
Your website needs a place to live on the web, and the quickest way to reserve your web address is to find a web host. There are a variety of cheap web hosting providers, and each one has a specialty. Look for a web host that offers a price that fits your budget, has the web tools you need to create your envisioned website, and provides support when you need help or have problems.