RCAC stands for "Row and Column Access Control", which is one of the features introduced in release 7.2 to DB2 for i (SQL).
Since the release announcement I have seen many questions asking what it is and how to implement it. IBM must have seen the same and have created a new Redbook called: Row and Column Access Support in DB2 for IBM i. You can download it here.
To accompany the release of the Redbook is this brief video of Mike Cain, one of the members of the DB2 for i Center of Excellence team, giving an overview of RCAC.