Answer:
"Marshalling" is roughly the same as "serialization", a mechanism many languages use to take classes, objects and values from a running program and convert them into a (normally binary) format for persistence (write to file or database) or for sending over a network.
A stream is a computer science concept meaning that bits are transmitted between a sender and a receiver one chunk at a time, like when sending a file over a network or writing to the hard drive.
A marshal stream would be a stream with marshalling data.
Marshalling isn't used in Javascript. The JSON serialization format can be considered a similar concept, but JSON differs from marshalling in the sense that JSON is only meant for serializing data, while marshalling in an object oriented language would also transmit the class definition and object methods.
Marshalling isn't really required in Javascript, as there is no compilation stage, and you can get the same effect by using eval() on a plain string representation.