Well for starters you only really care about the serializable interface if you are planning on writing objects to file and back (I think it's a necessary for MASON checkpointing, but I could be wrong about that).
The idea is that if you implement Serializable (and if you extend Mason Geometry you do) is that you need to make sure all the fields of your Agent class are also serializable (writable to file). This is a bit confusing since Serializable is really a marker interface for some behind the curtain java stuff rather than your classic method interfaces.