Your terminology is a little fuzzy there. But in JPA (not just Hibernate), a Generator is a mechanism for automatically assigning a primary key value when persisting a new object. Once persisted, that key's property value is accessible just like any other Entity property. Generators come in several varieties depending on where they look to determine what the next assignable key value is.
Some DBMS's have their own internal sequence generators also. So when defining a JPA Entity key that way,
you should use Generator type SEQUENCE.
Some people, when well-known sources tell them that fire will burn them, don't put their hands in the fire.
Some people, being skeptical, will put their hands in the fire, get burned, and learn not to put their hands in the fire.
And some people, believing that they know better than well-known sources, will claim it's a lie, put their hands in the fire, and continue to scream it's a lie even as their hands burn down to charred stumps.