Over exposure of hibernate pojo

Well I have recently started reading up on Hibernate so my knowledge is very raw. I read somewhere that you should not expose your hibernate pojo classes directly on your application rather you should create classes which represents Pojo classes on your application. It's like custom classes which use only some or all of the fields of the Pojo classes.

Can someone put some light onto this as to how in an application we can stop the over exposure of Pojos or what is the correct way of using Pojos and custom classes which are returned from the server.