Serialize means to deterministically flatten-out all the atomic parts of an object such that the parts can be written and then re-read without losing information. (Think the transporter on Star Trek.)
Swarm provides two ways to do this: in boxes and by wire.
HDF5
A library called HDF5 (http://hdf.ncsa.uiuc.edu/HDF5) provides former capability. HDF5 is a high-performance, portable, random access binary format for storing data. Swarm's HDF5 interface is in some sense like hiring competent professional movers: you point them at your stuff and they pack it efficiently into boxes and label everything in a clear way. They even write down how the room was arranged.
Various packages support HDF5.
Lisp format
Objects can also be represented in textual expressions suitable for sending over a byte stream.
Both mechanisms can be customized or extended by users in an agent's implementation.