Activation and Passivation is appilicable for only Stateful session bean and Entity bean.
When Bean instance is not used for a while by client then EJB Container removes it from memory and puts it in secondary storage (often disk) so that the memory can be reused. This is called Passivation.
When Client calls the bean instance again then Container takes the passivated bean from secondary storage and puts it in memory to serve the client request. This is called Activation.
Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.
In few words, "marshalling" refers to the process of converting the data or the objects inbto a byte-stream, and "unmarshalling" is the reverse process of converting the byte-stream beack to their original data or object. The conversion is achieved through "serialization".
The purpose of the "marshalling/unmarshalling" process is to transfer data between the RMI system.