In Oracle PL/SQL, the term REVOKE refers to removing or "un-granting" a privilege.
For example, if you revoke the UPDATE privilege from a user, that user will no longer be able to update tables or other objects in their schema.
If the UPDATE privilege is revoked from a role, user's who have been granted that role will no longer be able to update tables or other objects in their schema (assuming they don't have the UPDATE privilege enabled through any other grant or role).
This example demonstrates revoking the UPDATE privilege from the user "bob" on the sales table:
REVOKE UPDATE ON sales FROM bob;
If public (all users) had been granted privileges on sales and you wanted to completely revoke those privileges, you could execute the following REVOKE statement:
REVOKE ALL ON sales FROM public