Oh, I have no idea what those are. I think it's just the identifier MySQL gives to the foreign key constraint. With the various classes added to the Hibernate Config, you can have Hibernate create your database tables for you.
The nice thing is that the log files actually show you the scripts. In fact, I think if you do create(true,false) or something like that, it spits out the script in the log file, but doesn't actually connect to the database and create the tables, so you can see exactly what SQL, and it is SQL, that Hibernate will use. It's a great feature if you can do a 'top down' database design.