Proxies are classes generated dynamically by Hibernate to help with lazy loading. For instance, if you have a Cat class, Hibernate will generate a proxy class that extends Cat.
If you get an uninitialized instance of this proxy, essentially all its fields will be null except the ID because Hibernate has not yet hit the database. Now the first time you will call a method on this proxy, it will realize that it is not initialized and it will query the database to load it’s attributes. This is possible because the dynamically generated class overrides the base class’s methods and adds this initialized/uninitialized check.
Now assume that your Cat class is not a proxy and that it has a father association, when you load a Cat object, Hibernate will need to load all it’s attributes. So if you load a Cat object, Hibernate will also need to load its father and the father’s father and so on. Using proxies enable Hibernate to only load the required instances.
Cat cat1 = (Cat) session.load(1);
Cat cat2 = (Cat) session.load(2);
Cat cat3 = (Cat) session.load(3);
cat1.meow(); // this will cause Hibernate to run a query to load cat1's data
cat2.meow(); // this will cause Hibernate to run a query to load cat2's data
// After this cat3 is still an uninitiated proxy because it has not been used
batch-size is another feature of Hibernate that, in most instances, help dealing with lazy loading. Basically the idea is that Hibernate keeps track of the uninitialized proxies and when one of the needs to be initialized, a single query will be executed to load up to batch-size proxies (instead of just one proxy/query)
Cat cat1 = (Cat) session.load(1);
Cat cat2 = (Cat) session.load(2);
cat1.meow(); // if batch-size >= 2, cat1 and cat2 will be loaded in a single query
cat2.meow(); // no query will be executed here