serialVersionUID is a field to define the version of a particular class while seriializing & deseriializing.. consider a scenario where you have a class Employee which has 3 fields which has been in production for some time (meaning there may exist many serialized versions of employee objects), when you update the class to include (say a 4th field) then all the previous class (which are serialized) cannot be casted or de-serialized to the new one & you’ll get an exception.
to avoid this issue, you can use the serialVersionUID field to tell JVM that the new class is in fact of a different version (by changing the serialVersionUID).
@Farmor & @Tom Jefferys said pretty much the same thing, but with an example things look a lot simpler.