Often when switching hosts or signing up at new host you’ll have to set your domain’s name servers to your new website host’s name servers. This step is necessary because it allows your new host’s servers to receive and serve requests for your website from your visitor’s browsers. However the Domain Name System (DNS) is such that it takes time for this new mapping between your domain name and your host’s servers to propagate across the Internet.
Depending on your new host’s DNS servers and your Internet Service Provider it can take anywhere from a few hours to a few days before you will be able to access your website via the domain name. If, in the meantime, you want to upload files to your website and test it you will need to edit your HOSTS file to manually map your domain name to your web host’s IP address.