Surrogate Keys

Surrogate keys are for dimension tables 

A surrogate key is an attribute that is created to uniquely identify rows in a dimensiontable. It does not come from a source system; it is created expressly for the dimensional schema.

Surrogate keys for dimension tables serve two important purposes:

1. They make it easier to track history.  They allow the dimension to capture changes to something, even if the source does not.  Absent a surrogate key, this would be difficult; the primary key of the dimension would be concatenation of natural keys and type 2 attributes.
2. They make it easy to join to the dimension.  The dimensions' surrogate keys appear in fact tables as  foreign keys.  They allow the fact to be joined to the appropriate dimension values, without having to use a multi-part key.