It's
Java's way of writing objects to disk.
Example:
When you stop or restart an application in the
Tomcat application server, Tomcat will serialize all of the session objects and save them to disk.
When the app starts up again it reads them from disk and loads them back into memory. This allows you to restart the server or an application without interrupting people's sessions.
As Ulf mentioned: This won't occur unless all the objects bound to session implement Serializable and are serializable. Something like a socket or a database connection, for example can not be serialized to disk.