Optimistic locking is a technique where you allow your application to perform updates hoping there won't be any simultaneous updates from another process, hence optimistic. Pessimistic locking is the technique where you assume there will be simultaneous updates from another process so you lock the data you are changing before any changes take place.
In Hibernate, optimistic locking is provided fairly seemlesly. You add a version field to your table and a version element to your mapping file. When you update a Hibernate object Hibernate will check the version values match. If they do the update is successful, and the version is incremented, if they are not Hibernate throws a StaleObjectException and you know that something else has updated your data.
Pessimistic locking is provided by specifying a LockMode when you get an object from the database. If you get your object with the LockMode.UPGRADE for example Hibernate will use database specific SQL to lock the record (if your database supports it) such as "select ... for update". Now nothing can update that record till you are finished.