Yes this is how custom domains work. You have 2 choices, you can either purchase a custom domain/hosting through webflow if you have a free webflow account and it will allow you to choose your domain. You will need to buy your domain separately though and point the servers to webflow. The other, probably easier way, is if you have a paid webflow account with export, export the site and sign up for bluehost. Then you can just upload your site once you set it up with your domain name. Anytime you want to make changes you will have to export again and re-upload. Feel free to PM me if you need help with bluehost.
If you are new to webflow please watch all the tutorial videos at http://tutorials.webflow.com/