A SalesForce object can have a field of type called "lookup".
This field is used to by a record in a SalesForce object to reference a record in another SalesForce object.
The value of this field could be a string in which case it would be the record id of the other record or it could be an object in which case it would contain the other Object's external field name and its value in that other record.
Doing lookup/reference by external id is very useful specially for insert/upsert operations as otherwise one has to maintain record id of each of the records in the other Object.
Camel SalesForce component supports lookup by record id.
There is no support for the second type that is the external field name and value type.
See here for more information on this
Section "Upserting Records and Associating with an External ID"