Because virtual machines are files, you can back them up by making a copy.
Although virtual machines run other programs and manipulate files, they are still files themselves. Because of this, you can back them up by making a copy to external media and restore the virtual machines by copying back.
Unless you specify a file location for a virtual machine when you create it, Fusion saves the virtual machine package to a default location, which can vary. See Default File Location of a Virtual Machine.