The xtrabackup binary does not have any functionality for restoring a backup. That is up to the user to do. You might use rsync or cp to restore the files. You should check that the restored files have the correct ownership and permissions.
The datadir must be empty before restoring the backup. Also it’s important to note that MySQL server needs to be shut down before restore is performed. You can’t restore to a datadir of a running mysqld instance (except when importing a partial backup).
Example of the rsync command that can be used to restore the backup can look like this:
$ rsync -avrP /data/backup/ /var/lib/mysql/
As files’ attributes will be preserved, in most cases you will need to change the files’ ownership to mysql before starting the database server, as they will be owned by the user who created the backup:
$ chown -R mysql:mysql /var/lib/mysql
Note that xtrabackup backs up only the InnoDB data. You must separately restore the MySQL system database, MyISAM data, table definition files (.frm files), and everything else necessary to make your database functional – or innobackupex can do it for you.