A key (somtimes also called an index) is an access path to your database table. Keys can be unique or could be a primary key.
Keys can be imported. Keys are used by the NHibernate template to define access via factory methods.
If your table does not have a primary key, the table will be considered to be invalid. The NHibernate template will not generate any code for this table.
An index or key can be defined in the Key user interface:
Give the key a valid name and move the columns that you want in the key from the available columns listbox to the selected columns listbox.