Understanding oracle SCN
According to the Glossary from the Oracle Documentation


  System Change Number. A database ordering primitive. The value of an SCN is the logical point in time at which changes are made to a database.


There is lots of info you can easily Google


http://oracledbascriptsfromajith.blogspot.com/2009/05/understanding-scnsystem-change-number.html
http://www.adp-gmbh.ch/ora/concepts/scn.html


Even the DBA StackExchange already has questions around the SCN


How do I find my current SCN?
RMAN-06054: media recovery requesting unknown archived log, Where is SCN kept?
Commit vs Fast Commit vs Commit Cleanout in Oracle Database
more...