This is security feature that allows you to set which user's passwords can be cached on the RODC.
When a user successfully logs on to the RODC, the password can be cached by the RODC to speed up logons in the future, i.e. it doesn't have to consult another DC to process the logon.
The policy allows you to set which user accounts are allowed to be cached on the RODC, and which are not.
Have a read of this technet article for more info :
http://technet.microsoft.com/en-us/library/cc730883%28WS.10%29.aspx
