Typically in a database you have many users.

The owner shcema that owns the objects, usually decides the privileges given to other users on the objects it owns.

Lets call the schema owner "OWNER" and suppose we have another user in the database called "READER".

For the user "READER" to be able to view the data in a table owned by "OWNER", the owner has to grant the reader select priviliges.

Similarly the owner can grant insert,update, delete, etc privileges on a certain object to other users in the database.

To take away a certain privilege from a user

Unless a user has select privileges on an object owned by another schema, or that object has a synonym created, the user will not be able to access that object.