What is Serializable? What does this mean?

Possible Duplicates:
Do Hibernate table classes need to be Serializable?
What does Serializable mean?

public class ExampleEntity implements Serializable
{
     @Id
     private long id;
     private int fieldInt;
     private long fieldLong;
     private String fieldString;
 }

I am looking after JPA tutorials. I am able to understand the basic concepts, but in all the tutorials they have added, this serializable. What is the purpose of this? How does that help me? Any suggestions Please?