SAP systems include a reduced set of security features, which cover the SAP authorization concept and user authentication based on passwords. SNC is a software layer in the SAP Netweaver system architecture that provides an interface to an external security product offering stronger authentication methods, by encryption and by single sign-on mechanisms allowing SAP customers to extend SAP system security beyond the built in set of features shipped with SAP.
Keep in mind SNC is not a security product by itself. It only provides an interface to external security products which must implement any desired functionality in a manner defined in the standard interface GSS-API V2 (Generic Security Services Application Programming Interface Version 2). SNC uses this interface to communicate with an external security product (usually a library).