It is a method to replace (drop in) serialisation with both authenticated encrypted streams (normal connections) and unauthenticated (or anonymous) encrypted streams (bootstrap). So this is a library that forces use of a key pair or pre-computed key per connection. So it will be used to ensure no injection or manipulation attacks between nodes.
Easy way to think is that is changes serialise(data) with serialise(data, KEY), Where key is the actual key of the node we talk to. In this case the node will decrypt and parse with the key for the connection.
So encrypts, serialises and authenticates node to node transmissions. We added a decent readme in the docs to help get to the detail a bit more, http://maidsafe.net/secure_serialisation/master/secure_serialisation/index.html
Hope that helps. It is a handy wee wrapper around some nacl encryption (more accurately libsodium) methods by Daniel Bernstein which gives us a high degree of comfort the crypto primitives are correct.