A hosts file is a file on your computer, which allows you to change an IP address of a site and allowing you to view a site from another computer before the rest of the world can see it. Its like changing the DNS of a site, but only for your computer. This allows you to test features of your sites on new Webair servers before making the new server or site live.
On Windows, the file is located in your Windows install directory, such as:
On Apple OSX/Mac computers, and on all Unix/Linux systems , the file is located here:
< /etc/hosts >
In most cases you need to have Administrator / root access to a server in order to change this. Once changed you also usually need to restart your browser for the changes to be made properly.