ERNA originally was the acronym for Erasmus Remote Network Access. Nowadays it is the name used to describe a wide area of ICT-related services. The most apparant is your so-called ERNA-ID. This combination of username and password is used to identify you on most of the schools systems. You can check at www.erna.nl what services are offered.
Note: in case you forget your password you can request a new one at the SSC ICT Servicedesk in T3 or H2-01.