A user data file is a special key in the metadata service that holds a file that cloud-aware applications in the guest instance can access. For example, one application that uses user data is the cloud-init system, which is an open-source package from Ubuntu that is available on various Linux distributions and which handles early initialization of a cloud instance.
You can place user data in a local file and pass it through the
--user-data <user-data-file> parameter at instance creation.
$ nova boot --image Ubuntu-14.04 --flavor 100 --user-data mydata.file