What feature in Salesforce is needed to restrict access to that object?

Hi,

Sharing rule is nothing but extending the permissions given in the Owd criteria for a bunch of users. You can assign this to roles,single user,or a group of users. Sharing rule will extend access in one direction means
1) If u give OWD as private< you can change it to read , read/write , read/write/transfer.
2)If u give OWD as read< you can change it to read/write , read/write/transfer.
3)If u give OWD as read/write< you can change it to read/write/transfer.
4)If u give OWD as read/write/transfer.< you cannot change.