WhitewallManager is a whitelist manager. It aims to be a web based administration tool for administrators using a default-deny approach to the security of the network they are responsible for.
Default-deny is a superior model for network security as compared to default-allow, which is how the security model of most local area networks is modeled. Default-deny disallows all but access granted to resources which you specifically allow.
The advantage to this is that any new resources which turn out to be bad are blocked by default.
Default allow disallows nothing and bad things are blocked as they are discovered. This means that you are exposed to any bad things which you don't know about.
The reason most networks are modeled on default-allow is that default-deny is hard to manage.
What WhitewallManager aims to accomplish is to make default-deny and whitelists manageable with a sensible and efficient work flow. Currently it's a nightmare to manage a default-deny network.