"serializing" means that the raw bytes of an object's data (=the state of that object) are used to store that object's state for example to a database or file or to send it over a network and so on. Moreover the process of serialization and deserialization can be adapted to your needs.
So a serialized form of an object can be described as its state when it is not active in memory. Of course it should be possible to deserialize it later because otherwise the serialization wouldn't make much sense.
Hope that helps for a brief overview ;-)
subject: what is meant by saving an object's state to a sequence of bytes