Properties provide information about, as well as a way to search for, documents and objects in your portal. For example, you might want to create an Author property so users can find all the documents or objects created by a particular user.
When you add documents to the portal, the portal maps source document fields to portal properties according to mappings you specify in the Global Content Type Map, the particular content type definition, the Global Document Property Map, and any content crawler-specific content type mappings.