Complex sentence: A Surrogate key in a relational database is a unique identifier for either for an entity in the modelled world or an object in the database; it is "not" derived from application data.
Split into simpler sentences: A Surrogate key in a relational database is a unique identifier for either an "entity" in the modelled world or an "object" in the database. The Surrogate Key is "not" derived from application data.